Published Documents on Endangerment Causes- begins 1967
of Torreya taxifolia in Florida
Compilation by CONNIE BARLOW, October 2023
From environmental trigger hypotheses of multiple pathogens to
focus on a single fungus and back to climate change stress as ultimate cause
• Phase 2: Search for an Environmental Cause - begins 1995
• Phase 3: Focus on Fusarium sp. as Lethal Pathogen - begins 2010
• Phase 4: Fungal Endophytes Can Be Beneficial - begins 2016
• Phase 5: Fusarium torreyae in "all seeds" confirms core mutualism - begins 2018
• Phase 6: Mutualists Become Pathogens When Stressed by Climate Change - begins now
VIDEO posted Nov 2023: At timecode 9:06 the group's founder, Connie Barlow, begins a presentation of the long and shifting history of scientific speculation and (sometimes faulty) assumptions about the ultimate cause(s) of this ancient conifer's sudden demise in its tiny historical range in Florida.
She begins by reciting, with commentary, the "Purpose and Summary", as presented on this webpage. At timecode 53:30 she takes the remaining 14 minutes to scroll through the entire list (with excerpts) of the 50+ papers.
Purpose and Summary
PURPOSE:• Since 2003, I have been analyzing published papers that offer and/or test hypotheses on WHY FLORIDA TORREYA, a glacial relict, suddenly succumbed to disease(s) in its tiny native range in the 1950s.SUMMARY of the Six Phases:
• Until July 2023, I was unaware that, ca. 2016, agricultural research scientists initiated a PARADIGM SHIFT in how they regard microbial endophytes that is, fungi and bacteria living within plant tissues but not causing disease.
• This scientific shift in understanding draws attention to the fact that PLANTS HAVE A MICROBIOME and that fungal endophytes within plant seeds, the SEED MICROBIOME, are a sure sign of mutualism rather than disease.
• Microbial mutualists can, however, shift into a pathogenic lifestyle if their host is too environmentally stressed to produce seeds. More precisely, when the symbionts lose the opportunity to use seeds for their own "vertical transmission" of descendants into next-generation plants, they resort to the less secure mode of "horizontal transmission." Creating oozing cankers on stems or fruiting bodies on leaves are their only outlets for launching asexual propagules into the world.
• I therefore created this largely chronological list of scientific papers and excerpts to encourage others engaged in helping this endangered tree (professionals in the U.S. Fish & Wildlife Service and the two lead botanical gardens in Georgia) to modify their policies and methods in accordance with the new paradigm.
• A paradigm shift apparently is necessary in order for the tens of thousands of torreya seeds produced annually in the two ex-situ groves in northern Georgia to be offered once again for distribution. Results of the FOIA I submitted to the USF&WS in 2018 confirmed that ex situ seeds have been left undocumented and undistributed. (Seeds are recalcitrant, so they cannot be stored dry nor frozen.) The shut-down in seed distribution apparently owes to the fear of spreading a Fusarium species that has been documented as cause of lethal stem cankers in the wild population left in Florida. But the same Fusarium has also been documented unsymptomatically in "all tissues and seeds" in northward plantings. The new paradigm would shift the interpretation of the Fusarium from exclusively pathogenic to a mutualistic symbiont when the habitat itself is favorable for seed production.
• With such a paradigm shift, the ex-situ plantings would be useful for far more than simply "preventing extinction" and "genetic safeguarding." Their massive seed production could once again play a role in working toward full "recovery" (and thus de-listing) via "assisted migration" poleward in "experimental plantings", pursuant to the new regulation adopted July 2023.
PHASE 1 (observe pathologies and identify disease agents) began in the 1960s. Why did this once-abundant subcanopy species of evergreen conifer suddenly begin to die back to its roots in the small "historical range" of northern Florida? And while new stems continued to sprout from the root crowns, they would always eventually die back. Thus seed production virtually ended by the 1960s. What were the proximal causes of the disease(s)? And was there an underlying ultimate cause that provoked the pathologies?NORTHWARD DOCUMENTATIONS OF TORREYA GUARDIANS CAN HELP:
PHASE 2 (search for an environmental cause): Environmental stress that provoked disease expression in weakened plants was a reasonable hypothesis for this left-behind glacial relict. In 1995 the first paper delineating a half-dozen possible causes of environmental stress was published. Owing to a paucity of quantitative historical data (especially on temperature changes in the ravine habitats), the researchers could not point to a specific environmental shift as the likely ultimate cause of the multiple diseases injuring the species.
PHASE 3 (focus on Fusarium sp. as lethal pathogen) began in the 2010s. Enhanced genetic and morphological tools enabled plant pathologists to parse the FUSARIUM genus of fungi from fewer than a dozen recognized species worldwide into several hundred species today. Researchers used these tools to declare a canker-causing type of fusarium on torreya stems as a species distinct from fungal isolates that previously had been classified as the globally ranging Fusarium lateritium. The name chosen, Fusarium torreyae, along with its history of sudden arisal and deadliness, gave rise to speculations that the disease was exotic although that remains unproven to this day. No attempt was made by the researchers to ascertain a possibly larger geographic range and additional host species of the newly distinguished Fusarium pathogen. (However, in 2023 a paper was published that lists Fusarium torreyae as one of the Fusarium isolates on citrus trees near Gainesville, FL.)
Once Fusarium torreyae was distinguished from the globally ranging Fusarium lateritium, the risk of a possibly exotic pathogen spreading northward supplied new reasons for botanical garden staff in Georgia to remain hostile to "assisted migration" northward, and thus to actions already underway by citizens collaborating as Torreya Guardians. An additional perceived risk factor put a virtual end to harvesting and distribution of tens of thousands of torreya seeds produced annually in the two ex situ orchards in n. Georgia that are "safeguarding" the wild genetics. That new risk factor derived from the discovery that Fusarium torreyae was not only in "all tissues" of Florida torreya but even in the SEEDS produced by unsymptomatic plants in the ex situ northward groves.
PHASE 4 (fungal endophytes can be beneficial) became prolific in academic journals around 2016, having origins in the previous decade. Coming to the fore were review papers on the importance of fungal and bacterial "endophytes" found within plant tissues. Previously, all non-plant cells and genes found within plant tissues (other than roots) were assumed to be pathogenic, although sometimes not inducing any symptoms of disease. New research, however, overturned those assumptions. Plant stems and leaves (not just roots) actually require functioning fungal and bacterial microbes in order to be healthy and to deter pathogens. Just as human health is now known to require the health of our intestinal microbiome, review papers on the importance of fungal and bacterial endophytes acknowledge the PLANT MICROBIOME as important to all plant species thus far analyzed.
While some of the review papers pertaining to the plant microbiome mentioned the Fusarium genus as including species beneficial to the tissues of their hosts, none looked into the question of whether Fusarium torreyae may be playing a beneficial role in its host. But how could it? How could Fusarium torreyae be beneficial to its host plant if it unquestionably was associated with stem cankers?
PHASE 5 (Fusarium torreyae "in all seeds" confirms core mutualism) depended on papers, beginning around 2018, that focused on the mutualism between plant and microbes found within the seeds of plants. This became known as the SEED MICROBIOME. It also became apparent that some microbial mutualists that assisted the plant in seed protection and germination would morph into pathogens if a plant were so stressed that seeds could no longer be produced.
Consider: If a fungal or bacterial microbe could no longer launch its offspring securely into the next-generation of plants via "vertical transmission" within plant seeds, its only alternative would be to launch its propagules randomly via fruiting bodies on leaves or as liquids oozing from stem cankers. Canker disease thus came to be seen as a form of "horizontal transmission" for fungal and bacterial endophytes that normally provided benefits to the plant in return for a share of photosynthates. Quite simply, cankers form when a microbe determines that its "ship" is sinking.
PHASE 6 (mutualists become pathogens when stressed by climate change) is a logical extension of the "environmental stress" implications of Phase 5 especially for a "glacial relict" plant that already had to survive in place during the post-glacial rise in temperatures long before anthropogenic global warming ensued. Crucial, too, is that only in recent years has there been ample evidence (in the western states) that native insects and pathogens can be triggered by warming and drought to suddenly shift into killing vast acreages of native trees in wild habitat. See: "Climate Change Risks to Global Forest Health: Emergence of Unexpected Events of Elevated Tree Mortality Worldwide". Thus, the suddeness of torreya stem deaths in Florida mid-20th century can no longer be assumed as a strong indicator for an exotic origin of an identified pathogen, such as Fusarium torreyae.
As yet, there are no papers or even documented communications suggesting that anyone other than myself is applying the new SEED MICROBIOME research toward helping Florida torreya recover. Indeed, the argument for prioritizing torreya's extinction risk over the small risk of disease propagation is strengthened by two papers published in 2023. They conclude that (1) the strongest case for mutualism is for those microbes routinely found within seeds and (2) Fusarium is one of the four most-recognized fungal genera entailing seed microbiomes in general. This latter finding is surprising because traditionally recognized Fusarium species are associated with some of the greatest losses of commercial plant crops, especially in the tropics and subtropics.
THE BOTTOM LINE: Even with no further research specific to Fusarium torreyae, one can reasonably conclude this fungal endophyte is almost certainly a "facultative mutualist" that undergoes a life transition into pathogenicity when one or more other causes preclude Torreya taxifolia from engaging in seed production. A good place to start is to read the 2023 paper, "Integrating seed microbiome knowledge into restoration and ex situ conservation of native Australian plants", by Allison A. Mertin et al., 2023, Australian Journal of Botany.
Torreya Guardians have documented two findings that should amplify the choice of turning to "assisted migration" for extinction prevention versus continuing to prohibit seed distribution as a means to minimize the risk of disease spread. The two documentations are:
1. Achieving seed production in Ohio, along with documenting seed production in old horticultural plantings in North Carolina.
2. Finding no disease symptoms in states north of Georgia. (See the Torreya Guardians "Historic Groves" documentation.)
There is thus strong grounding to support an official shift in management policy. This is especially appropriate given (a) the July 2023 regulatory change that authorized placement of "experimental populations" at sites beyond "historical range" and (b) the upcoming December 2023 50th anniversary of the Endangered Species Act.
In 2023 Congress made it clear that preventing extinction of 99% of the listed species is nothing to brag about. Ultimately, recovery and thus de-listing is the measure of worth of how the 1973 Act is being carried out.
• BACK TO 6-PART TABLE OF CONTENTS
Phase 1 - begins 1967
Observe Pathologies and Identify Disease Agents
• 1967 - "Stem and Needle Blight of Florida Torreya", by S.A. Alfieri, Jr, A.P. Martinez, and C. Wehlburg, Florida State Horticultural Society.EXCERPTS: ... The species is affected with a fungus disease contributing to a progressive state of decline which threatens the existence of the species both in its natural habitat and wherever it is grown. Trees in all stages of development are affected. The symptoms of the disease are characterized by a small, circular, chlorotic spot on the needles which turns light brown until the entire needle becomes necrotic. Infection of the stem follows, accompanied by abscission of the needles, leaving twigs with small tufts of new growth at the tips.... In a few cases, cankered stems were observed to occur on affected and dead Torreya trees. Moreover, it was also observed that, in general, the most severely affected trees were those that were growing UNSHELTERED IN FULL SUNLIGHT .... The stem and needle blight disease of Torreya taxifolia appears to be incited by a fungous causal agent which implicates Physalospora and Macrophoma in a manner not yet clearly understood.• 1985 - "Fusarium lateritium causing needle spots on Torreya taxifolia in Florida", by N. E. El-Gholl, Plant Disease Vol. 69: 905.
• 1987 - "Leaf and Stem Disease of Torreya taxifolia in Florida", by S.A. Alfieri, C.L. Schoulties, K.R. Langdon, and N.E. Gholl, Plant Pathology Circular, No. 291, Florida Dept. Agriculture and Consumer Services.EXCERPTS: ... The purpose of this circular is to provide: 1) an update on microorganisms associated with leaf and stem disease of T. taxifolia (root-inhabiting microorganisms are not included in this discussion), 2) illustrations of some of these leaf and stem microorganisms, 3) to provide inspectors, growers, and the public information on the decline of the Florida Torreya.
Five genera of fungi were documented on "stems and needles."] Only Fusarium lateritium infected needles in the absence of wounds, and symptoms developed within three days. Infected needles fell off within 2 weeks after inoculation, and sporodochia of the fungus were evident on the lower side of the needles along the two bands of stomates. This constitutes added information and is the first documented report of proven pathogenicity. However, F. lateritium was infrequently isolated.... Invasion by the microorganism progresses through the needle into the stem, followed by abscission of the needles. As various degrees of defoliation become manifest, the stems appear essentially bare with small tufts of new growth at the tips. Fungus fruiting structures occur on stems and the undersurface of needles. Severely diseased trees are marked by much needle and stem necrosis and severe defoliation. Often, entire branches will turn yellow, then brown. Swollen gall-like symptoms with roughened bark on Torreya branches were observed by the senior author on the University of Florida campus in Gainesville.
... Every effort should be made to save and to replenish this rare tree by all available means, to ascertain the most suitable habitat, encourage research on the stress factors affecting tree survival and distribution, and on various microorganisms that have been found associated with the stem and needle blight which is contributing to its demise.
Phase 2 - begins 1995
Search for an Environmental Cause
• 1995 - "The Catastrophic Loss of Torreya taxifolia: Assessing Environmental Induction of Disease Hypotheses", by Mark W. Schwartz, Sharon M. Hermann, and Christoph S. Vogel, Ecological Applications.INTRODUCTION: The Florida torreya (Torreya taxifolia) is endemic to ravines along the east side of a 35 km stretch of the Apalachicola River in the central panhandle of Florida and adjacent Georgia. Formerly a common subcanopy to small canopy tree, this endangered conifer is on the verge of extinction in the wild as a result of a catastrophic decline during the 1950s and 1960s. The loss of T. taxifolia is thought to have been a proximal result of fungal pathogens (Godfrey and Kurz 1962), although no primary pathogen has been identified (Alfieri et al. 1967, 1984, Schwartz 1990). Continuing efforts to find a virulent pathogen, including viral or bacterial agents, have yielded no likely candidates (R. Doudrick, J. Entry, and J. Muchovej, pers. comm.). The lack of an obvious pathogen has led to speculation that environmental stress rendered T. taxifolia more susceptible to native pathogens (Toops 1981, Barnes 1985, U.S. Fish and Wildlife Service 1986, Schwartz 1990). We assess the efficacy of several hypotheses proposed to explain the decline of T. taxifolia.• 1995 - "The relationship between an endangered North American tree and an endophytic fungus", by Julie C. Lee, Xianshu Yang, Mark Schwartz, Gary Strobel, and John Clardy, Chemistry and Biology. PDF.
EXCERPTS: ... In lieu of an obvious pathogen, numerous hypotheses have been proposed to explain the loss of T. taxifolia. These hypotheses can be grouped as biotic and abiotic changes in the environment. BIOTIC hypotheses include: (1) as yet undetected introduced pathogens, (2) increased disease transmission through altered behavior of pathogen vectors, and (3) a virulent pathogen that decimated the population but is now in low abundance and virtually undetectable. These three hypotheses, while remaining viable, have little supportive evidence and no potential for experimental assessment at the present time; they will be discussed no further in this paper.
In contrast, hypotheses related to ABIOTIC changes in the environment can be assessed both circumstantially through historical evidence and indirectly through experimentation. ... Abiotic hypotheses include: (1) moisture and temperature stress, (2) changes in soil chemistry, associated with disruption of hydrologic regimes when upland topsoils were plowed in the 1950s contributing to plant stress, and (3) fire suppression, which may have altered ravine habitats and rendered T. taxifolia more susceptible to disease.
... If T. taxifolia is limited by warm temperatures a, slight increase in temperature may render it susceptible to native pathogens. This species may be an early casualty of global warming. [Table 2]
CONCLUSIONS: Historical observations, coupled with physiological and growth responses to moisture, temperature, and soil nutrients, indicate little supportive evidence to suggest that T. taxifolia has been subjected to unusual stress in its environment during, or since, the time of the population crash. While we cannot rule out the possibility that moisture, temperature, or soil nutrient stress contributed to inciting disease in T. taxifolia, the argument that one of these concurrent environmental changes single-handedly incited a decline is very weak. As a result, no specific management recommendations to facilitate recovery, or to stem potential disease inception, were derived from this research.
Our laboratory results suggest that increasing light for trees should facilitate growth response and increase vigor among the trees in the field. In contrast, field tests failed to show a relationship between light and growth or vigor. Together these results suggest that other physiological pathways, such as root function, may be limiting growth in the wild and need to be examined. Our results relating to the fire suppression hypothesis suggest that this hypothesis remains, at least, plausible. In light of these results, additional hypotheses, including the presence of an unknown pathogen or the effects of air pollution, must be pursued.
EXCERPTS: BACKGROUND: The Florida torreya (Torreya taxifolia) began a catastrophic decline in the late 1950s, and is now the rarest tree in North America for which a full species designation has been established. The trees have common plant disease symptoms, but the reason for the decline has never been identified....• 1996 - "Occurrence of Pestalotiopsis microspora on Torreya taxifolia in Florida", by M.W. Schwartz, D.J. Porter, S.M. Hermann, and G. Strobel, Plant Disease
INTRODUCTION: The Florida torreya (Torreya taxifolia Arn. or stinking yew) is the rarest tree in North America for which a full species designation has been established. This yew-like conifer is endemic to 400 km2 of ravine slopes along a short stretch of the eastern bank of the Apalachicola River in northern Florida and adjacent Georgia. In this small region, it was a common subcanopy tree, but during the late 1950s it began a catastrophic decline, and within a decade, no sexually mature adults remained. The decline continues, and the fewer than 1500 remaining trees have been placed on the Federal endangered species list. Why are the torreyas disappearing? The most prevalent symptoms of diseased trees are needle spots, needle death and stem cankers, but no virulent pathogen has been described. Lack of an associated pathogen has led to speculation that environmental changes resulting from unregulated forestry practices on adjacent lands over the past 80 years rendered the trees vulnerable to attack from native pathogens.
...Like most plants, torreya serves as the host for a large number of intimately associated microorganisms such as endophytic fungi.... Endophytic fungi are characterized by their ability to invade the intercellular space in the living tissues of plants, and inside virtually every plant there are several species of endophytic fungi. The nature of the association between endophytic fungi and their hosts ranges from symbiotic, to near neutral, to parasitic.... Our efforts to characterize the endophytic fungi of the Florida torreya and their biosynthetic products have led to the hypothesis that the tree's decline results from a pathogenic relationship with the endophytic fungus Pestalotiopsis micospora.
RESULTS AND DISCUSSION: ... To test whether Koch's third postulate applies requires deliberately infecting torreyas with laboratory cultures of P. microspora. T. taxifolia limbs were inoculated with P. microspora by placing a small agar block containing the fungus into a slit on a symptomless torreya limb. The trees were maintained under normal growth conditions in the Montana State University Plant Growth Facility. Within two weeks, all 20 artificially inoculated trees exhibited canker-like growths with necrotized brownish tissue on their stems along with chlorosis on their terminal leaves symptoms comparable to those observed in declining trees in the wild....
... While more than 30 different endophytic fungi were isolated from wild Florida torreyas during this investigation, only Pestalotiopsis micospora was found to be consistently associated with the tree. This organism, based on the available data, is a plausible causative agent for the Florida torreya's decline. Since hosts and parasites that have been associated for a long time usually co-evolve to a relationship in which the parasite does not kill the host, P. microspora's pathogenicity may indicate a recent association with torreya. One possibility is that it has been introduced via the large scale cultivation of non-native pines on the land surrounding the torreya's habitat. Alternatively it may have long coexisted with torreyas but became pathogenic as the torreya was stressed by environmental disturbances brought on by intensive forestry practices in the region. Whatever the detailed mechanism, the involvement of P. microspora in the decline of the Florida torreya deserves further consideration. The case for its involvement is increased by the coupling of chemistry with Koch's postulates, and the identification of the fungal metabolites that both directly cause disease symptoms and could possibly give P. microspora a competitive advantage over other fungi....
CONCLUSIONS: The filamentous fungus, P. microspora, has an endophytic-pathologic relationship with T. taxifolia. The fungus resides in the inner bark of symptomless trees, and physiological or environmental factors could trigger its pathological activity. P. microspora produces the phytotoxins pestalopyrone, hydroxypestalopyrone, and pestaloside which give rise to the disease. Pestaloside, which also has antifungal activity, could reduce competition from other fungal endophytes within the host.
ABSTRACT: Isolates from diseased T. taxifolia were identified as P. microspora. Trees across the indigenous populations of T. taxifolia in Florida USA, were examined for the fungus. Of the 62 examined, isolates were found on 56 plants. Twig age was not related to the probability of infection and no geographic distribution pattern to the disease was found. It is concluded that almost the entire population of T. taxifolia was infected by P. microspora.• BACK TO TABLE OF CONTENTS
Phase 3 - begins 2010
Focus on Fusarium sp. as Lethal Pathogen
• 2010 - Update of the Florida Torreya Recovery Plan, by U.S. Fish & Wildlife Service.EXCERPTS (pp. 5-6): Identify pathogen(s) responsible for the decline. This is an ongoing action that goes back to 1967 (Alfieri et al. 1967). The authors observed that the stem and needle blight disease of T. taxifolia appears to be incited by a fungus causal agent implicating Physalospora and Macrophoma. Alfieri et al. (1987) isolated six other fungi from leaves and stems of T. taxifolia. Lee et al. (1995) isolated more than 30 different endophytic fungi, but consistently, Pestalotiopsis microspora (a fungus that resides in the inner bark of symptomless T. taxifolia trees) was found on diseased trees. They concluded that the pathological activity of this fungus could be triggered by physiological and/or environmental stress.• 2010 - "The Decline of Florida Torreya: An Endemic Conifer on the Edge of Extinction", by Jason A. Smith and Aaron Trulock, University of Florida School of Forest Resources and Conservation, research paper.
Herman and Schwartz (1997) studied the pathogenicity of Scytalidum sp. They inoculated both needles and stems causing needle spots and necrosis, but couldn't provide evidence that it was the cause of the original decline of T. taxifolia. Soil-borne pathogens, such as Phytophthora sp., Pythium sp., Rhizoctonia solani and Sclerotium rolfsii have been reported on T. taxifolia. At present, Dr. Lydia Rivera (Univ. of Puerto Rico, 2009) is conducting a soil-borne pathogen survey, emphasizing the detection of Phytophthora spp. She isolated 102 fungi from TSP, FL, and Corps property, GA. Of the trees surveyed, 48% had root necrosis and stem cankers. She is designing a pathogenicity test associated with potential disease outbreaks.
Dr. Jason Smith (Univ. of Florida) is conducting an above-ground plant pathogen study. He isolated numerous fungi from cankers and consistently found an undescribed Fusarium sp.; he is working with a specialist in Japan to describe the new Fusarium species. According to Smith (2010, pers. comm.), "inoculation experiments with seedlings and larger potted torreyas have provided ample evidence that Fusarium is the causal agent" for the current population decline; the cause of the initial decline remains unknown. When plants are inoculated with Fusarium, it leads to canker development, lesions, and mortality (Smith 2010, pers. comm.). He proposes to elucidate the disease biology, as well as conductive epidemiological factors and treatment. Aaron Trulock is a graduate student under Smith and will be doing his research on the biology of canker disease of the T. taxifolia.
EXCERPTS: Considered a common tree in its restricted habitat until just before WWII, by 1962 Florida torreya had declined so severely that the species was considered to be destined for extinction (Godfrey and Kurz, 1962). The decline of Florida torreya was first observed around 1938 (Alfieri et al., 1967). By the 1960s, no adult individuals could be found and the outlook for the species seemed very bleak (Alfieri et al., 1967). The rapid decline of the species was then attributed to an unknown fungal disease because of the abundance of leaf spots and stem cankers and the rapid nature of the decline (Godfrey and Kurz, 1962). Since then, Florida torreya has continued to decline and in addition to disease, has been subjected to changes in hydrology, forest structure, heavy browsing by deer and a loss of reproductive capability (Schwartz and Hermann, 1995). Despite these challenges, Florida torreya stems which have been killed by disease often re-sprout from the stump in a manner reminiscent of American chestnut following chestnut blight, although seed reproduction has been non-existent for decades in the wild (Schwartz and Hermann, 1999). Estimates show Florida torreya has declined 99% since pre-settlement population levels, from an estimated population of 357,500 individuals in 1914 to approximately 1,350 in the 1990s (Schwartz et al., 2000). It is believed that the population has declined further since 2000 to current estimates of 400–600 individuals (T. Spector, unpublished data, 2010).• 2010 - REVIEW: "Biogeography and Phylogeography of Fusarium: A Review", by Brett A. Summerell et al., Fungal Diversity.
Despite several attempts to conclusively determine the causal agent responsible, disease etiology has not been previously elucidated (Alfieri et al., 1967; Alfieri et al., 1987; El-Gholl, 1985; Lee et al., 1995; and Schwartz et al., 1996). In the first pathology studies conducted on T. taxifolia (Alfieri et al., 1967), it was noted that disease symptoms of leaf spots, needle necrosis, defoliation and stem lesions were common on native and cultivated T. taxifolia. Several pathogens were isolated commonly from symptomatic needles (Macrophoma sp., Rhizoctonia solani, Sphaeropsis sp. and Sclerotium rolfsii), however, no pathogens were isolated from cankered stems and Koch's postulates (proof of pathogenicity) were not demonstrated. About 20 years later, El-Gholl (1985) implicated Fusarium lateritium as a causal agent by demonstrating this species' capacity to cause leaf spots, however, the causal agent of the canker disease remained unknown. Alfieri et al. (1987) completed more pathogenicity studies with a Phyllosticta sp., Xylocoremium flabelliforme and F. lateritium. They also completed Koch's postulates with F. lateritium as a leaf spot pathogen, but the canker-causing organism remained elusive. In 1991, Schwartz et al. implicated Pestalotiopsis microspora as the causal agent of the canker disease, having isolated the pathogen from 56 symptomatic plants and completed Koch's postulates on 10 stems. However, no information was given on the canker development, morphology or ability to cause mortality. Typically Pestalotiopsis spp. are considered opportunistic pathogens (Sinclair, 2005). Lee et al. (1995) investigated the endophytic and pathogenic chemical attributes of P. microspora infection and artificial inoculations resulted in stem canker development, however, again no stem mortality was observed.
Subsequent studies by Hermann and Schwartz (1997) implicated a Scytalidium sp. due to frequent isolation from cultivated and naturally occurring Florida torreya. Inoculation attempts led to small lesions on needles, but cankers were not observed.
In addition to biotic causes of decline, researchers have looked into changes in soils, drought, global warming, sunlight exposure and fire regime as possible causes of decline (Schwartz et al., 1995). Some of these environmental changes are thought to have occurred because of the building of the Woodruff Dam along the Apalachicola River in 1957 (Schwartz et al., 1995), and changing land uses in the surrounding areas. However, none of these environmental hypotheses have been demonstrated as a cause of the decline. The rapid nature of the decline during the period of 1938 to 1945 and numerous observations of disease symptoms provides ample evidence that a pathogen, possibly non-native, was involved (Schwartz et al., 1995).
EXCERPTS: As genetic and molecular techniques have become more sophisticated, more widely available and more commonly applied, formerly functional species concepts, definitions and relationships are being stretched, with numerous new species being described. The species concept for Fusarium depends to at least some degree on those describing the species, and there is no universally accepted rule or guideline currently followed across the entire Fusarium research community. The Dictionary of Fungi (Ainsworth et al. 2001) notes that 500 species of Fusarium have been reported, although the number of common and well-described species is considerably smaller. The number of new species and their documentation should continue to increase as new ecosystems are explored and as the broad species concepts for some species are redefined more precisely. Thus, a continuation of the ongoing reassessment of Fusarium species definitions and their boundaries is needed to accurately define existing species, and to ensure that all fungi with Fusarium names warrant inclusion within the genus. • 2011 - REVIEW: "Fifty years of Fusarium: how could nine species have ever been enough?", by Brett Anthony Summerell & John F. Leslie, Fungal Diversity.
Morphological species concepts: Defining a species depends on differences in morphology between species, i.e., the species look different (Mayr 1963). This approach has been used by fungal taxonomists for over 200 years, is well known and understood, and is supported by extensive technical literature....
Biological species concepts: The biological species concept as put forth by Mayr (1940; 1963) considers "... species as groups of populations that actually or potentially interbreed with each other." There are practical problems in applying a biological species concept to many Fusarium species, as many strains of Fusarium are asexual and only rarely, if ever, produce a sexual stage, even under laboratory conditions. Nonetheless, the biological species concept has been used quite profitably for some groups, most notably the Gibberella fujikuroi complex (Leslie 1995).
Phylogenetic species concepts: The phylogenetic species concept (Taylor et al. 2000) is an even more recent development in mycology that can clarify many taxonomic difficulties, but if inappropriately applied or misinterpreted can lead to confusion. This species concept is especially useful for species that do not regularly produce a sexual stage. Phylogenetic species concepts utilize multiple markers, usually differences in DNA sequences of selected genes, and can yield quantitative measures of genetic relatedness. When working with phylogenetic species concepts alone, a common problem is where to draw the line between "species", i.e. "How different must two strains be to belong to different taxa?" In practice, many fungal phylogenetic studies rely on DNA sequences of one or two loci, from one or a few representative or well-characterized isolates. This process can result in problems that are best avoided by ensuring that enough loci and enough individuals are studied to obtain an accurate picture of the variation within the species while remaining true to the original definition of phylogenetic species, which relies on populations rather than individuals as the analytical unit.... We believe that before such groups are raised to species level solely on the basis of sequence differences that additional techniques that scan the genome for differences, or even better, that use a distinct species concept, should be employed to verify the existence and the significance of the observed sequence differences.
Biogeographic concepts in the origin of Fusarium: With the available evidence, it is difficult to identify a centre of origin for Fusarium. Members of the genus have been found in virtually all environments and are dispersed globally. The genus is thus likely to be ancient and could have appeared relatively early in the evolution of the ascomycetes. The diversity of the morphological and phylogenetic traits across the genus is consistent with this hypothesis. Taylor and Berbee (2006) calibrated a molecular clock for key groups of fungi, including Fusarium. Depending on the taxonomic group chosen as the calibration point the minimum age for Fusarium could be 110, 250 or 420 million years ago. All of these dates precede the major splitting and creation of the current land masses and, as such, provide support for the diversity and early spread of Fusarium species to all types of ecosystems on a global basis. Thus, climate and host(s) are likely to be informative about the origin of various Fusarium species as the geographic location from which they can be recovered.
Dispersal of Fusarium: There are almost as many dispersal processes in Fusarium as there are species, including human transport, which makes generalizations regarding dispersal difficult. Certainly there are long distance dispersal processes known today (Schmale et al. 2006) that could have contributed to the widespread dispersal of various members of the genus across millennia. When combined with anthropogenic dispersal of host plants and their associated products, e.g., soil, there is more than sufficient long-range movement to effectively mask much of the underlying biogeography of the genus. Long-range movement of Fusarium falls into three broad categories: air dispersal, spread in soil, and distribution with infected host plants.... Backhouse et al. (2001) argued that there are four different types of distribution patterns found in species of Fusarium: cosmopolitan, host-associated, climate-mediated, and anthropogenic.
Phylogeography of plant pathogenic species of Fusarium: Tracking form species often has been done to search for geographic origins of various pathogenic strains, e.g. F. oxysporum f. sp. radicis-lycopersici, which may have been imported to the United States from Israel (Rosewich et al. 1999) or F. oxysporum f. sp. cubense race 1 which appears to have been circulated worldwide with banana cuttings (Ploetz 1990). Without supporting evidence of the genetic identity or similarity of the strains involved, however, many of these reports are questionable. Instead, the first query should be whether the form species pathotype in question might have arisen locally.
Other exceptions to the O'Donnell et al. biogeographic hypothesis include F. werrikimbe, F. succisae, F. bulbicola and F. sterilihyphosum, all of which are associated with hosts that have proposed geographic origins that are outside the area predicted by the clade they are associated with (Adams 1955; Britz et al. 2002; Kvas et al. 2009; Nirenberg and O'Donnell 1998; Walsh et al. 2010). These exceptions have been attributed to anthropological dispersal of economically important plants and host jumps by the fungi (O'Donnell et al. 1998a), but also may be an artefact of the small number of available isolates of these species, which may not accurately reflect the primary geographic range or the most important host substrate. The source and manner in which the fungal isolates were recovered has a significant impact on the hypotheses that are developed based on them. If the isolates evaluated are primarily collected from agricultural ecosystems, then any biogeographic information may be masked by the anthropogenic distribution patterns caused by the distribution of agricultural germplasm. Consequently, artefacts will arise in and confuse the distribution patterns. We have long argued that it would be more appropriate to define biogeographical clades that attempt to explain the evolution of species within the genus based on isolates derived from natural ecosystems.
CONCLUDING REMARKS: Biogeographic surveys of non agro-ecosystems continue to prove their importance in understanding the taxonomy and evolutionary history of Fusarium. The application of robust phylogenetic analysis to Fusarium populations from non agro-ecosystems will continue to improve our understanding of the evolution of this remarkable genus.EXCERPTS:... The genus is well known for taxonomic difficulties and has been composed of anywhere between 9 and >1,000 species depending on the authority queried and the time when the question was asked. Definition and identification of species remains problematic, with problems at both ends of the scale when are too many things grouped into a single species, e.g. F. oxysporum (e.g. Laurence et al. 2011) and F. solani, and when have the separations gone too far, e.g. the 13 phylogenetic species within F. graminearum (O'Donnell et al. 2000) or the separation of F. brevicatenulatum from F. pseudoanthophilum (Amata et al. 2010). Morphological characters often are indicative, but may not be definitive, while molecular markers need to be checked against other characters since where to draw the line between species when only molecular markers are used is not always clear (Summerell et al. 2003). • 2011 - "A Novel Fusarium Species Causes a Canker Disease of the Critically Endangered Conifer, Torreya taxifolia", by Jason A. Smith, Kerry O'Donnell, Lacey L. Mount, Keumchui Shin, Kelly Peacock, Aaron Trulock, Tova Spector, Jenny Cruse-Sanders, and Ron Determann, Plant Disease, 7 pp. with photos.
... Fusarium is a genus of ascomycete fungi first described by Link (1809) as Fusisporium. Members of the genus are numerous and can be recovered from plants and soil worldwide as pathogens, endophytes and saprobes (Summerell et al. 2010). Members of the genus are notorious for their capabilities as plant pathogens, although work with native plants and soil in undisturbed areas suggests that the number of species not associated with known diseases may far outnumber those that cause disease. Most members of the genus produce an array of secondary metabolites, which vary widely in chemical form (Desjardins 2006).
... the recognition of biological species and the development of DNA technology radically changed the way species concepts were viewed in Fusarium, and indeed in all fungi. Species concepts in Fusarium have been discussed in some depth (Leslie et al. 2001), and most current workers now use a combination of morphological, biological and phylogenetic markers in a polyphasic approach based on a relative weighting of the available data and markers.... From 1998 onwards there was a proliferation of new species described; this was a consequence of the recognition that a number of the traditional morpho-species are species complexes composed of many species combined with the exploration of both crops and natural vegetation in poorly understood ecosystems.
... Fusarium can cause various types of diseases, including vascular wilts, head and seed blights, stem rots, root and crown rots and canker diseases, with some species capable of simultaneously causing multiple or overlapping disease syndromes depending on the host and the environment.... Root and crown rots are probably the most widespread type of disease caused by Fusarium species. In many cases the causal agents of these diseases may be difficult to determine, as they may occur in plants affected by other factors that are simultaneously infected by weak pathogens or saprophytes.
... Canker diseases involve the formation of lesions on stems and branches and most prominent on woody perennial species in horticulture and forestry. Pathogens such as F. circinatum, the cause of pitch canker in pine, and F. decemcellulare, which causes cankers on tropical fruit trees, are good examples of the pathogens in this category.... Chemical control measures have been spectacularly unsuccessful for Fusarium diseases and there are very few fungicides currently available that could be described as economically effective control strategies for these diseases.ABSTRACT EXCERPT: A canker disease of Florida torreya (Torreya taxifolia) has been implicated in the decline of this critically endangered species in its native range of northern Florida and southeastern Georgia. In surveys of eight Florida torreya sites, cankers were present on all dead trees and 71 to 100% of living trees, suggesting that a fungal pathogen might be the causal agent. To identify the causal agent, nuclear ribosomal internal transcribed spacer region (ITS rDNA) sequences were determined for 115 fungi isolated from cankers on 46 symptomatic trees sampled at three sites in northern Florida. BLASTn searches of the GenBank nucleotide database, using the ITS rDNA sequences as the query, indicated that a novel Fusarium species might be the etiological agent...
[Final paragraph]: Florida torreya faces numerous challenges to its future survival in its natural habitat. In addition to the canker disease, deer routinely cause damage to stems from antler rubbing. It is unclear whether they are attracted by the tree's aroma or seek out Florida torreya for some other unknown reason. Whether the wounds caused by deer serve as infection courts for pathogens, including the new Fusarium sp. (Fsp-1), is unclear and warrants further study. Additionally, since lesions on the larger plants in IE3 resulted in less stem girdling and no mortality, the host response to infection, particularly under different stress conditions, needs to be investigated. In addition to more research on the biology and management of CDFT, more work is needed to assess the various factors involved in decline of Florida torreya and how the species can be protected from extinction.
• 2012 - "Host Range and Biology of Fusarium torreyae, Causal Agent of Canker Disease of Florida Torreya, by Aaron J. Trulock, 55 pp., (master of science thesis, University of Florida).ABSTRACT: Florida torreya is an endangered, endemic conifer with a limited range near the Apalachicola River. The species began to decline in the 1950's and the population has plummeted from an estimated 375,000 to approximately 1,000. Since 1967 many investigations have tried to determine the cause, including pathological and environmental factors. The Torreya Guardians propose recovering the species through assisted migration, where the species would be "re-introduced" into the southern Appalachian Mountains, the potential "historic" range of Florida torreya. In 2010 a previously unknown pathogen was discovered on Florida torreya, which causes stem cankers and stem girdling. The potential host range was investigated via artificial inoculations. Species tested included conifers whose range overlaps with Florida torreya, other species from the Torreya genus, and conifers from the southern Appalachian Mountains. One species with an overlapping range, Florida yew (Taxus floridana); two other Torreya species, California torreya (Torreya californica) and Chinese nutmeg yew (Torreya grandis), and five species from the Appalachian Mountains, Fraser fir (Abies fraseri), Red spruce (Picea rubens), White pine (Pinus strobus), Table mountain pine (Pinus pungens), and Eastern hemlock (Tsuga canadensis) were found to be potentially susceptible. The effect of temperature on the growth, sporulation, and spore dissemination was tested, also. Growth was maximized at 25 C, sporulation was maximized at 20 C, however, there was no significant difference in spore dissemination among the temperatures tested. The necessity of wounds for infection was investigated; it was found that F. torreyae cannot infect leaf or stem tissue without the presence of wounds.• 2012 - "34. Fusarium Root and Stem Diseases", by Robert L. James, pp. 117-120 in USDA Forest Nursery Pests.
FINAL PARAGRAPH: Fusarium torreyae appears to have the ability to complete its lifecycle if moved to a new, cooler location and would not be limited by the new environment. The movement of infected T. taxifolia plants into the southern Appalachian Mountains may spread this pathogen to a new area, to stressed, injured susceptible hosts and create a new epidemic. What is the value in moving one species to potentially imperil others?
EXCERPTS: Fusarium diseases are caused by several Fusarium species, especially F. oxysporum, F. proliferatum, F. solani, and several species within the 'roseum' complex, including F. acuminatum, F. avenaceum, F. sambucinum, F. sporotrichioides, and F. equiseti. Recent evidence indicates that some highly virulent strains previously identified as F. oxysporum, should be classified as F. commune. Although all conifer and many hardwood species may be affected, Douglas-fir, pines, larch, and true firs are most susceptible to Fusarium diseases. Spruce is often damaged less and species of cedar and cypress are relatively resistant to Fusarium diseases.• 2012 - "Somatic embryogenesis, plant regeneration, and cryopreservation for Torreya taxifolia, a highly endangered coniferous species", by X. Ma, K. Bucalo, R.O. Determann, J.M. Cruse-Sanders, and G.S. Pullman, In Vitro Cellular & Developmental Biology - Plant.
... Fusarium species cause seedling root decay, stem cankers (including hypocotyl rot), seed decay, and wilt of some plant species within nurseries. Infected seedlings are often killed or have reduced growth and vigor. This pattern often results in increased numbers of culls and reduced survival after outplanting. Mortality is more common during the first growing season on bareroot stock and toward the end of the growth cycle on container seedlings. Losses can vary greatly depending on nursery management practices, seedling species, seedlot, geographic area, soil type, and environmental factors.
.... Many different plant seeds may be contaminated with Fusarium and may provide an important pathogen introduction source for nurseries.
ABSTRACT EXCERPT: Torreya taxifolia Arn., an ancient evergreen tree, is on the brink of extinction from attack by a fungal disease, recently reported to be caused by a novel isolate of Fusarium. We report the development of a somatic embryogenesis tissue culture system that can be used for cryogenic storage of T. taxifolia cultures and subsequent plant regeneration.... Germination of somatic embryos ranged from 64 to 82 %. Embryogenic tissue cultures from 30 genotypes representing seed from six mother trees were cryopreserved, and culture recovery was demonstrated after freezing. In contrast to many other coniferous tree seeds, the measured water potential (-MPa) of T. taxifolia megagametophyte tissue rose greatly during seed after-ripening. Duplication of this rise in vitro allowed development of somatic embryos to the cotyledonary stage.• 2013 - "Root and Soil-borne Oomycetes (Heterokontophyta) and Fungi Associated with the Endangered Conifer, Torreya taxifolia in Georgia and Florida", by Lydia I. Rivera Vargas and Vivian Negron-Ortiz, article in Vol 1 of a new journal titled Life: The Excitement of Biology (25 pp; Dr. Vivian Negron-Ortiz was USF&WS staff person and primary writer/signer of the 2010 and 2020 recovery plan updates.).
EXCERPTS and TABLES: ... Several Fusarium species have been shown pathogenic to T. taxifolia, by causing needle spots, i.e. F. lateritium Nees (El-Gholl 1985, Alfieri et al. 1987), and has been associated with root rot (Alfieri et al. 1984). More recently, a novel described species F. torreyae Aoki, Smith, Mount, Geiser, and O'Donnell, was demonstrated to be the causal agent of stem cankers (Smith et al. 2011). Until now, no Fusarium spp. has been demonstrated to cause cankers comparable to those observed in the field. In other conifer species, F. oxysporum Schltdl. Emend. Snyder and Hansen has been associated with seedling death and root rot (Viljoen et al. 1992).
... In this study, various Fusarium species (anamorph of Gibberella spp.) were isolated in both locations from roots and associated to bark, plant litter and dying trees. From Georgia, Fusarium oxysporum was isolated from roots and plant litter. This species has been associated with seedling death and with root rot in Pinus and Eucalyptus seedling in South Africa (Viljoen et al. 1992). Fusarium solani was also isolated from roots of dying trees at this site (Table 3). Based on macroconidia and chlamydospores morphology, none F. torreyae was isolated during this study (Aoki et al. 2013). Recently this species have been shown to cause Torreya canker disease in Florida (Smith et al. 2011). DNA analysis of the ITS rDNA region showed that Fusarium spp. isolated during this study were closed to F. subglutinans, F. oxysporum and Gibberella spp. clades (Figure 5). However, amplification of other more informative genetic regions such as RNA polymerase largest subunit (RPB1 and RPB2) was not employed during this study....
• 2013 - "Fusarium torreyae sp. nov., a pathogen causing canker disease of Florida torreya (Torreya taxifolia), a critically endangered conifer restricted to northern Florida and southwestern Georgia", by Takayuki Aoki, Jason A. Smith, Lacey L. Mount, David M Geiser, Kerry O'Donnell, Mycologia.EXCERPTS (in which the Fusarium is named and distinguished from closest Fusarium relatives): During a survey for pathogens of Florida torreya (Torreya taxifolia) in 2009, a novel Fusarium species was isolated from cankers affecting this critically endangered conifer whose current range is restricted to northern Florida and southwestern Georgia. Published multilocus molecular phylogenetic analyses indicated that this pathogen represented a genealogically exclusive, phylogenetically distinct species representing one of the earliest divergences within the Gibberella clade of Fusarium. Furthermore, completion of Koch's postulates established that this novel species was the causal agent of Florida torreya canker disease. Here we formally describe this pathogen as a new species, Fusarium torreyae.• 2014 REVIEW PAPER, "Systematics of key phytopathogenic Fusarium species: Current status and future challenges", by Takayuki Aoki, Kerry O'Donnell, and David M. Geiser, in Journal of General Plant Pathology
... The available data suggests these three fusaria can be distinguished by host range in that F. torreyae is known only from Florida torreya (Smith et al. 2011), F. lunulosporum has been isolated only from grapefruit (Gerlach and Nirenberg 1982) and wheat (Gert van Coller, A-L Boutigny, A Viljoen pers comm) in South Africa, and F. tucumaniae has been recovered only from soybean in Argentina and Brazil (Aoki et al. 2003, O’Donnell et al. 2010). The identification of F. torreyae as F. lateritium by El-Gholl (El-Gholl 1985) and subsequent confirmation by Paul E. Nelson (D. Geiser pers comm) illustrate the daunting challenge presented by overly broad morphological concepts of fusaria, especially when applied to species that produce only sporodochial conidia. We speculate that the reported needle blight of Florida torreya induced by F. lateritium in a pathogenicity experiment (Alfieri et al. 1987) also can be attributed to F. torreyae; however, no isolate from this study was accessioned so the identity of this pathogen cannot be verified. Other examples of misplaced Fusarium spp. pathogenic to trees are provided by the etiological agent of coffee wilt disease in Africa, F. xylarioides Steyaert, and vascular wilt of pigeon pea (Cajanus cajan (L.) Millsp.) in Africa and Asia caused by F. udum Butler. The latter two species were classified in section Lateritium by Booth (1971), but molecular phylogenetic analyses have clearly established that they are nested within the African clade of the Gibberella fujikuroi species complex (O'Donnell et al. 1998, Geiser et al. 2005, Lepoint et al. 2005)
... Most fusariologists included only fusaria that produce conidia that are mostly straight in F. lateritium (Wollenweber and Reinking 1935, Booth 1971, Gerlach and Nirenberg 1982, Leslie and Summerell 2006). However, Nelson et al. (1983) also included isolates from woody plants that produced macroconidia more curved in the midsection, which is consistent with his phenotypic identification of FRC L-212 (5 NRRL 54626 F. torreyae) as F. lateritium. We speculate that ElGholl's (1985) and Nelson's identification of FRC L212 F. torreyae as F. lateritium also was influenced by the fact that the latter species is broadly attributed to canker diseases on trees. Our discovery that FRC L-212 is F. torreyae, and not F. lateritium, was made during a multilocus phylogenetic assessment of F. lateritium clade species accessioned in the FRC and the CBSKNAW Biodiversity Centre. As posited by Leslie and Summerell (2006), our preliminary assessment suggests this clade comprises at least 15 phylogenetically distinct species, not including F. torreyae, which was resolved as a distinct monotypic lineage. Because the limited morphological characters produced by F. torreyae may pose difficulties in identifying it phenotypically, we recommend conducting nucleotide BLAST queries of GenBank, Fusarium-ID (Geiser et al. 2004) or Fusarium MLST (O'Donnell et al. 2010), using portions of EF-1a, RPB1 or RPB2 gene sequences to obtain a definitive identification.
ABSTRACT: This review is intended to provide plant pathologists and other scientists with a current overview of the most important Fusarium phytopathogens and mycotoxin producers. Knowledge of Fusarium species diversity and their evolutionary relationships has increased dramatically due to the application of multilocus molecular phylogenetics and genealogical concordance phylogenetic species recognition over the past 15 years. Currently Fusarium is estimated to comprise at least 300 genealogically exclusive phylogenetic species; however, fewer than half have been formally described. The most important plant pathogens reside in the following four groups: the F. fujikuroi species complex noted for Bakanae of rice, ear rot of maize, pitch canker of pine and several species that contaminate corn and other cereals with fumonisin mycotoxins; the F. graminearum species complex including the primary agents causing Fusarium head blight of wheat and barley that contaminate grain with trichothecene mycotoxins; the F. oxysporum species complex including vascular wilt agents of over 100 agronomically important crops; and the F. solani species complex, which includes many economically destructive foot and root rot pathogens of diverse hosts. Several other Fusarium phytopathogens reported from Japan and nested within other species complexes are reviewed briefly....• 2016 - "Two novel Fusarium species that cause canker disease of prickly ash (Zanthoxylum bungeanum) in northern China form a novel clade with Fusarium torreyae", by Zhou Xue, Kerry O'Donnell, Akayuki Aoki, Jason A. Smith, Matthe T. Kasson, Zhi-Min Cao, Mycologia.
EXCERPTS: Canker disease of prickly ash (Zanthoxylum bungeanum) has caused a decline in the production of this economically important spice in northern China in the past 25 y. To identify the etiological agent, 38 fungal isolates were recovered from symptomatic tissues from trees in five provinces in China. These isolates were identified by conducting BLASTN queries of NCBI GenBank and phylogenetic analyses of DNA sequence data ... Results of these analyses suggested that 30/38 isolates belonged to two novel fusaria most closely related to the Florida torreya (Torreya taxifolia Arn.) pathogen, Fusarium torreyae in Florida and Georgia. These three canker-inducing tree pathogens form a novel clade within• 2016 -"First report of fusarium crown and root rot on Torreya grandis caused by Fusarium oxysporum species complex in China", by Zhang Chuangqing, Shuya Zhang, X.L. Chen, Q.Q. Qi, H.Z. Lou, Plant Disease.
Fusarium here designated the F. torreyae species complex (FTOSC).
...Molecular clock estimates place the divergence of the FTOSC in the mid-Eocene, 40 Mya (O’Donnell et al. 2013), but it remains an open question whether this clade first evolved in the Old or New World. Furthermore, it remains to be determined whether F. torreyae is native to North America and restricted to T. taxifolia. Surveys for F. torreyae on Torreya endemic to China are warranted because it is the modern area of diversity of this genus (Li et al. 2001) and because the putative Asian origin of the CDZB pathogens could indicate that the most recent common ancestor of the FTOSC evolved in Asia.
EXCERPTS CONTINUE: ... Several species of Fusarium are capable of causing cankers on woody plants, and mixed infections frequently occur. For example, a particular FSSC species occasionally was found co-occurring with F. torreyae in cankers on Florida torreya (Torreya taxifolia) (Smith et al. 2011). Although both species could induce cankers, F. torreyae is considered to be the primary pathogen due to increased virulence and consistent isolation from a large number of cankers. By contrast the available data indicates the FSSC taxon should be regarded as an opportunistic (J. Smith pers comm).
ABSTRACT EXCERPT: Torreya grandis cv. Merrillii is an important economic forest crop with a price as high as $75/kg in nut markets throughout humid regions of South China. In 2009, sporadic occurrence of crown rot disease was recorded in Shaoxing City, which produces 80% yields of T. grandis in China. In 2014, nearly 37% of orchards and 4.5% of trees were affected in Shaoxing. Symptoms were crown rot characterized by light-brown discoloration of the cambium, with brownish black necrotic areas which appeared on the roots and often advanced to the collar on most plants. Diseased trees showed reduced vigor and chlorosis on the foliage and eventually died.... To our knowledge, this is the first report of T. grandis crown and root rot caused by fungus belonging to the F. oxysporum species complex worldwide. Further work is needed to determine the clade of F. oxysporum to which the isolates pathogenic to T. grandis belong.• 2017 - "Wilt, Crown, and Root Rot of Common Rose Mallow (Hibiscus moscheutos) Caused by a Novel Fusarium species", by S. L. Lupien, F. M. Dugan, K. M. Ward, and K. O'Donnell, PLant Disease.
EXCERPTS: ... the Hibiscus pathogen was nested within the Fusarium buharicum species complex (FBSC).... The FBSC was strongly supported as sister to a clade comprising members of the F. lateritium and F. torreyae species complexes (Fig. 3A to D), which are primarily canker-inducing tree pathogens (Smith et al. 2011; Zhou et al. 2016).• 2018 - "Heterothallic sexual reproduction in three canker-inducing tree pathogens within the Fusarium torreyae species complex", by Xue Zhou, Kerry O'Donnell, Hye-Seon Kim, Robert H. Proctor, Gail Oehring, Zhi-Min Cao, Mycologia.
EXCERPT of ABSTRACT: Fusarium zanthoxyli and F. continuum are sister taxa that are the etiological agents of canker disease of prickly ash (Zanthoxylum bungeanum) in northern China. These two pathogens, together with F. torreyae, the causal agent of canker disease of the critically endangered conifer Florida torreya (Torreya taxifolia) from northern Florida and southwestern Georgia, constitute a novel clade, the F. torreyae species complex.... Genotyping of 34 - 40 progeny from the F. zanthoxyli and F. continuum crosses confirmed that they were the products of sexual reproduction. However, only 36% of the progeny in the F. torreyae cross were recombinant, which was roughly half of the nonparental progeny expected with three markers segregating.• 2018 - "Symptomatic Citrus trees reveal a new pathogenic lineage in Fusarium and two new Neocosmospora species", by M. Sandoval-Denis et al., Persoonia
ABSTRACT: The diversity of fusaria in symptomatic Citrus trees in Greece, Italy and Spain was evaluated using morphological and molecular multi-locus analyses ... Three new Fusarium species are described ... belonging to the newly described F. citricola species complex and the F. fujikuroi species complex...."• 2018 - "New Fusarium species from the Kruger National Park, South Africa", by Marcelo Sandoval-Denis, Wijnard J Swart, and Pedro W. Crous, MycoKeys.
EXCERPTS: ... Currently, more than 1,400 Fusarium names are listed in the Index Fungorum and MycoBank databases.... Positive ecological interactions between fusaria and Citrus spp. have been recorded for species formerly included in Fusarium, i.e., Microcera coccophila (Syn Fusarium coccophilum) and Microcera larvarum (Syn Fusarium larvarum), successfully employed as biocontrol agents against citrus fruit attacking armoured scales. While Fusarium taxonomy is actively changing, with numerous species being described each year mostly based in molecular phylogenetic approaches, just a handful of studies deal with the distribution of Fusarium spp. in Citrus, and there is scant data for the Mediterranean basin.... More studies are therefore needed on these new taxa in order to elucidate their host range, specificity, and global distribution, as well as their potential impact on the Citrus industry.
EXCERPTS: The genus Fusarium includes a vast number of species, commonly recovered from a variety of substrates including soil, air, water and decaying plant materials; being also able to colonise living tissues of plants and animals, including humans; acting as endophytes, secondary invaders or becoming devastating plant pathogens (Nelson et al. 1994). In addition to their ability to colonise a multiplicity of habitats, Fusarium is a cosmopolitan genus, present in almost any ecosystem in the world, including human-made settings such as air and dust in the indoor environment or even in hospitals (Perlroth et al. 2007; Aydogdu and Asan 2008; Pinheiro et al. 2011).... Little attention has however been given to the occurrence of non-pathogenic fungal species, including Fusarium spp. in root microbial communities (Zakaria and Ning 2013; Jumpponen et al. 2017; LeBlanc et al. 2017), while comprehensive DNA sequence-based surveys have been directed mostly to the study of highly relevant and abundant rhizosphere fungal genera such as Trichoderma Pers., Verticillium Nees or mycorrhizal fungi (Zachow et al. 2009; Bent et al. 2011; Ruano-Rosa et al. 2016; Saravanakumar et al. 2016).• 2019 - "Antiplasmodial and Cytotoxic Cytochalasins from an Endophytic Fungus, Nemania sp. UM10M, Isolated from a Diseased Torreya taxifolia Leaf", by Mallika Kumarihamy et al., Molecules.
... The analysis of the FBSC included sequences of EF-1α, RPB1 and RPB2 loci from 18 isolates representing 10 taxa, including members of the Fusarium torreyae T. Aoki, J.A. Sm., L.L. Mount, Geiser & O’Donnell species complex (FTYSC) and Fusarium lateritium Nees species complex (FLSC) as outgroup (Fig. (Fig.2).2).
... The three Fusarium species, described here, were not associated with any visible symptomatology on their hosts. However, they cannot be ruled out as pathogens since they were not assessed for pathogenicity against the sampled plants nor any other putative host species at the same locations. Likewise, it is unknown if these fungi exert any beneficial or deleterious effect on their ecosystems. These are important unsolved questions that need further evaluation. However, as shown by phylogenetic analyses, each of the three new species was in close genetic proximity with well-known plant pathogenic Fusarium spp. on their respective species complexes, which could suggest a potential pathogenic role.
LAST PARA: This study is a new example of how easily new Fusarium spp. can be found when mycological studies are directed to neglected natural ecosystems of minimal anthropogenic disturbance (Phan et al. 2004; Leslie and Summerell 2011; Summerell et al. 2011; Burgess 2014, Laurence et al. 2015). Although irrelevant for some researchers, finding and properly describing new species, regardless of whether they have little or no pathogenic or mycotoxigenic potential, is of utmost importance to improve our understanding on the diversity, biogeographic and phylogeographic patterns of such a complex and heterogeneous genus as Fusarium. In addition, this study remarks on the significance and need to further stimulate the exploration of conserved, non-manipulated natural environments (supersites) and their potential impact on biodiversity research on the fungal kingdom.
EXCERPTS: Plant-like metabolic pathways in apicoplasts serve as promising targets for antimalarial drug discovery. We have screened a number of plant pathogenic fungal extracts with phytotoxic activity for their antiplasmodial activity. Some of these extracts yielded compounds with good antiplasmodial activity. As part of this program, we investigated endophytic fungi isolated from a diseased leaf of cultivated Torreya taxifolia. One of the fungal extracts showed potent phytotoxic activity and selective antiplasmodial activity. This fungus (UM10M) was identified as a species of the genus Nemania (Xylariaceae). Some members of this family have been identified as endophytes and plant pathogens.... It showed weak suppressive activity but its high toxicity to animals would preclude it as a potential malaria drug lead.
• 2019 - "Three novel Ambrosia Fusarium Clade species producing clavate macroconidia known (F. floridanum and F. obliquiseptatum) or predicted (F. tuaranense) to be farmed by Euwallacea spp. (Coleoptera: Scolytinae) on woody hosts", by Takayuki Aoki, Jason A. Smith, and 5 coauthors, MycologiaEXCERPTS: Ambrosia Fusarium Clade (AFC) mutualists are carried in specialized cavities called mycangia by female Euwallacea beetles, which farm them in galleries they construct in the xylem of their woody hosts and use them as a source of nutrition for themselves and their larvae (Hulcr and Stelinski 2017). This mutualism has attracted considerable attention by agricultural scientists because several xyleborine Fusarium-farming Euwallacea beetles cause dieback and death of economically important hosts, including Chinese tea (Camellia sinensis), cacao (Theobroma cacao), rubber tree (Hevea brasiliensis), citrus (Citrus spp.) (Brayford 1987), and avocado (Mendel et al. 2012). The discovery of invasive Asian Euwallacea farming Fusarium associated with wilt and dieback of avocado (Persea americana), as well as diverse woody hosts in managed urban landscapes and native forests in the United States, Israel, Australia, Mexico, and South Africa, during the past decade has spurned intensive research efforts focused on understanding the genetic diversity of these exotic mutualists...• 2019 - Audio podcast interview of a botanical garden staff person.
The present study reports on multilocus molecular phylogenetic analyses and detailed phenotypic data that were used to distinguish three unnamed AFC species ... Herein, these species are formally described as Fusarium floridanum (AF-3) farmed by E. interjectus on boxelder (Acer negundo) in Gainesville, Florida ... Of these, Fusarium floridanum and F. obliquiseptatum are known to cause limited cankers associated with beetle attacks on boxelder and avocado, respectively.
Due to the significant threat the invasive Euwallacea-Fusarium mutualists pose to urban landscapes, native forests, and the avocado industry worldwide, extensive pathogen surveys and multilocus molecular phylogenetic analyses conducted over the past 6 years led to the surprising discovery of 15 additional phylospecies within the AFC.... Although the majority of the fungus-farming Euwallacea are not known to cause widespread damage, the likelihood that they might switch symbionts offers the real potential of creating more aggressive and economically destructive
Euwallacea-Fusarium pest-pathogen associations.
• 2020 - "Detection method for Fusarium torreyae the canker pathogen of the critically endangered Florida torreya, Torreya taxifolia", by Tyler J. Dreaden, Tania Quesada, Jason A. Smith, Forest Pathology.EXCERPT OF ABSTRACT: ... Recently, a canker disease caused by Fusarium torreyae was identified as the primary cause of Florida torreya decline. Efforts to restore and preserve the species in situ and ex situ are hampered by lack of pathogen-free planting stock, and there exists an interest in methods to verify pathogen presence in seeds and seedlings prior to collection and transport for planting. This paper presents a new species-specific diagnostic method that enables detection of F. torreyae and may allow for conservation programmes to ensure germplasm is free of the pathogen prior to planting.• 2020 - "Fighting Fusarium Pathogens in the Era of Climate Change: A Conceptual Approach", by Salme Timmusk et al., Pathogens.
EXCERPTS: The pathogen that might have caused the rapid decline in the 1950s is not known, but the Florida torreya is currently being affected by a canker disease caused by F. torreyae and it is hypothesized that this disease might be responsible for the species' rapid decline (Aoki et al., 2013; Smith et al., 2011). An ex situ conservation strategy was initiated with one cutting from each of 150 trees sampled and placed in four botanical gardens (Schwartz, 1993). Ex situ collections are ongoing (Smith et al., 2011), with the Atlanta Botanical Garden (Atlanta, GA) continuing to acquire new accessions and maintaining a large collection. Many of the botanical gardens were unable to maintain the collections, and most of the samples are currently found only at the Atlanta Botanical Garden. There is therefore a need to have ramets of these accessions at other locations to ensure their preservation. This is hampered by the necessity to ensure that the canker pathogen, F. torreyae, is not found in seed or seedlings that will be sent to the new locations where that pathogen is currently not found. This study was undertaken to develop a rapid and inexpensive PCR-based method to identify F. torreyae from culture to help screen samples for the pathogen before propagules of T. taxifolia are moved to new locations.
... Current T. taxifolia conservation/restoration efforts include ex situ collections. While this will help maintain the genotypes, the movement of the plants is a pathway for also spreading the canker pathogen to new locations. The assay developed here can be used to screen T. taxifolia plants or seed before they are moved to new locations and thus limit the spread of the damaging canker pathogen that could affect other hosts in new environments (Trulock, 2013).
EXCERPTS: Fusarium head blight (FHB) caused by Fusarium pathogens is one of the most devastating fungal diseases of small grain cereals worldwide, substantially reducing yield quality and food safety. Its severity is increasing due to the climate change caused by weather fluctuations.... Fusarium is one of the most renowned genera in the Fungi kingdom. It includes, in a broad sense, a large number of morphologically and phylogenetically diverse fungi, commonly found as air-, soil-, or water-borne saprobic organisms, but also found in dead or living plant material as endophytes or epiphytes. Many Fusarium spp. are effective plant pathogens or secondary invaders of worldwide concern.... Based on the perceived scientific and economic importance, Fusarium spp. were recently included in the top 10 globally most important genera of plant pathogenic fungi.• 2020 - "Biological control of Fusarium wilt in crop plants using non‐pathogenic isolates of Fusarium species", by Suresh Patil and s. Sriram, Indian Phytopathology.
Plants harbor different species- and cultivar-specific microbial communities that grow as epiphytes and endophytes in the rhizosphere and phyllosphere. The phyllosphere microbiome is composed of various microorganisms but bacteria are the most abundant cellular organisms in the phyllosphere community, and the most effcient colonizers are present in numbers between 106 and 107 cells cm 2 of leaf tissue. The bacteria in phyllosphere are often vertically transmitted as endophytes and they form patchy assemblages on plant surface called biofilms or microbial consortia. While the rhizosphere and endo-rhizosphere have been thoroughly studied for centuries, less is known about the drivers that influence the phyllosphere, i.e., any aerial part of the plant, such as stems, leaves, leaf ears, or flowers. This is a significant limitation as phyllosphere microbiome is of critical importance for dispersal of Fusarium pathogens. Although Fusarium spp. can survive several years on plant debris, even if located deep in the soil, the pathogens themselves, however, develop only on the upper parts of living plants. Hence, the review focuses on the phyllosphere, which has a large potential for the design of beneficial microbial biofilm layers.... Competitive exclusion of pathogens by the broader phyllosphere community may play an important role in plant pathogen resistance.
Biofilm formation is an environmental microbiology concept, denoting microbial cell adaptation to multicellular lifestyle. The temporal sequence of events in biofilm formation starts with planktonic cell attachment, followed by microcolony formation and cell detachment. Biofilm formation maximizes nutrient delivery to all viable cells and increases resistance to environmental stress factors. The attachment is initiated by exuding extracellular polysaccharide material that entraps cells and debris within a glue-like matrix.
Despite importance of plant heath for forestry and agriculture, its meaning is not fully conceptually developed. The reductionist approach is rooted in investigating small parts of pathosystems. Interactions with pathogens as well as beneficial microbes have been studied for years in different academic fields. The reductionist approach is rooted in investigating small parts of pathosystem, and the broad scientific field is currently highly fragmented. In particular, interactions of plants with pathogens and with beneficial microbes have been studied for years in different academic fields. Microbial community interaction networks have been studied by microbial ecologists, volatiles by chemical ecologists, environmental signals by climate and soil scientists, and interactions with harmful microorganisms are in the domain of plant pathologists. As a result, over the years, each of the fields has developed its own approach and language. It is clear that our current challenge is to strengthen the connections between ecological and agricultural studies.... As a general practice, most of the plant-pathogen interaction research has been performed under a few static conditions that do not allow capturing the pathosystem dynamics as it occurs in the nature.
ABSTRACT: Many species of the genus Fusarium resemble each other morphologically. In many cases morphological differentiation is difficult, molecular tools are used. Pathogenic as well as non-pathogenic isolates have same habitat and colonize plant root system with equal measure. Since non-pathogenic isolates resemble pathogenic isolates in their nutritional and abiotic requirements, many attempts have been made to utilize them as biocontrol agents to manage Fusarium wilt diseases. Although these isolates colonize plants, they do not induce disease symptoms. They are primarily soil inhabiting in nature; once introduced they remain in the soil for very long time providing significant and consistent disease control. Herewith we give a review and summary of different reports of non-pathogenic Fusarium in different cropping systems, mode of action (antibiosis, plant growth promotion and induced systemic resistance), molecular basis of bioassay and identification, environmental conditions, different formulations, cross protection as well as effects on non-target crops.
EXCERPTS: The taxon Fusaria is a complex one and more than one hundred formae speciales have been described. It represents rhizosphere microflora that are plant pathogens or saprophytes. Even the pathogenic isolates survive as saprophytes, and once they reach the corresponding host they infect and enter into parasitic life style. Non-pathogenic isolates are capable of colonising root surfaces and protect even susceptible varieties from the highly virulent pathogenic isolates. From asymptomatic roots, avirulent Fusarium isolates can be isolated.
• 2020 - UPDATE OF THE FLORIDA TORREYA RECOVERY PLAN, U.S. Fish & Wildlife Service.FULL TEXT of "Ongoing Action: Identify pathogen(s) responsible for the decline", p.5.• 2021 - "Antimalarials and Phytotoxins from Botryosphaeria dothidea Identified from a Seed of Diseased Torreya taxifolia", by Mallika Kumarihamy and 8 coauthors (including Edward M. Croom, Jr, descendant of the EuroAmerican who first reported the existence of this tree, Hardy Bryan Croom), Molecules.
2020: An ongoing project at ABG [Atlanta Botanical Garden] is focused on determining whether the fungal pathogen (see 2011, 2013 Smith & collaborators findings) has infected outplanted material at Vogel State Park and Smithgall Woods State Park, GA and if it has moved into surrounding trees. A preliminary inspection of trees at Vogel State Park revealed fungal cankers on outplanted T. taxifolia material, and fungal infections on surrounding Tsuga caroliniana and Carya sp. Fungal material from these trees has yet to be sequenced to determine if the species infecting these trees is Fusarium torreyae.
2019: Kumarihamy et al. (2019) investigated endophytic fungi isolated from a diseased leaf of cultivated T. taxifolia; compounds isolated showed potent in vitro activity against P. falciparum D6 and W2 strains (protozoan parasite that causes malaria).
2013: A systematic survey (soil-borne pathogen survey of roots, soil and plant litter associated with T. taxifolia) was conducted from three sites at TSP, Florida, and one site in Decatur, Georgia. About 102 fungi were isolated: 27 isolates (26%) were from TSP and 75 (74%) from Georgia. All T. taxifolia trees sampled showed moderate to severe levels of decline; 48% had root necrosis and stem cankers. Composition of fungal community included plant pathogens, lignin and cellulose decomposers, endophytes and saprophytes.
2011, 2013: Smith et al. (2011) conducted an above-ground plant pathogen study. They isolated numerous fungi from cankers and consistently found Fusarium torreyae (Aoki et al. 2013), as possible etiological agent. According to the studies, when Torreya plants were inoculated with F. torreyae, it leads to canker development, lesions, and mortality. Dr. Smith's lab identified F. circinatum in a canker on one of the lower branches of a permanently planted individual at the Bok Tower Garden (Bok Garden; P. Lynch, Bok Garden, 6/03/2020, pers. comm.).
1997: Herman and Schwartz (1997) studied the pathogenicity of Scytalidum sp. They inoculated both needles and stems causing needle spots and necrosis, but couldn't provide evidence that it was the cause of the original decline of T. taxifolia.
1967: This ongoing action goes back to 1967 (Alfieri et al. 1967). The authors observed that the stem and needle blight disease of T. taxifolia appears to be incited by a fungus causal agent implicating Physalospora and Macrophoma. Alfieri et al. (1987) isolated six other fungi from leaves and stems of T. taxifolia. Lee et al. (1995) isolated more than 30 different endophytic fungi, but consistently, Pestalotiopsis microspora (a fungus that resides in the inner bark of symptomless T. taxifolia trees) was found on diseased trees. They concluded that the pathological activity of this fungus could be triggered by physiological and/or environmental stress.
EXCERPTS: ... As part of our program to search for new antimalarials from plant pathogenic fungi, we investigated FUNGI FROM SEEDS OF A DISEASED TORREYA TAXIFOLIA Arnott. (Taxaceae).... The decline of the native population during the recent past has been attributed to both abiotic and biotic causes, including fungal diseases. Several fungi have been isolated from diseased T. taxifolia and some of them have been shown to cause leaf spots and canker disease in healthy plants. For this study, SEEDS of T. taxifolia were collected from a tree with disease symptoms cultivated on the Biltmore Estate in Asheville, North Carolina.• 2023 - "Root Rot Disease of Torreya grandis Caused by Fusarium fujikuroi in China", by L. Zheng et al., Plant Disease
From fragments of a surface-sterilized SEED, several ENDOPHYTIC FUNGI were isolated. An EtOAc extract of the broth of one of these fungi grown in potato-dextrose liquid medium showed phytotoxic and antiplasmodial activities. This fungus (UM124) was identified as Botryosphaeria dothidea (Botryosphaeriaceae) by DNA analysis. Members of the family Botryosphaeriaceae (Botryosphaeriales, Ascomycota) cause leaf spots, fruit and root rots, and cankers in a variety of hosts, and B. dothidea has specifically been isolated from a large number of diseased and healthy woody plants, including many economically important crops. A Botryosphaeria sp. strain has previously been isolated from T. taxifolia LEAVES infected with needle-spot disease.
... This is the first report of the isolation of a fungus producing phytotoxins from the SEEDS of diseased T. taxifolia. B. dothidea might play a significant role in decreasing the population of the endangered T. taxifolia.... Dothilactaene B was isolated from the active fraction, which showed moderate in vitro antiplasmodial activity with high selectivity index. In spite of this activity, its instability and various other biological activities shown by related compounds would preclude it from being a viable antimalarial lead.
EXCERPT: Torreya grandis is an evergreen plant endemic to China and widely grown in Southern China.... From 2018 to 2020, typical root rot symptoms of T. grandis were found in plantations in Huangshan and surrounding areas of Huangshan, Anhui province, China. About 15 to 32% of root rot disease incidence was recorded. Diseased plants were observed with symptoms such as yellow to brownish leaves without lesions and later drying, and rotten roots that looked dark brown and eventually led to the death of the diseased plant. The root rot symptomatic plants were collected in June of 2020.... Eight isolates with similar morphology were isolated from single spores.... According to the culture and conidial characteristics, the isolates were tentatively identified as Fusarium species.... From symptomatic roots, the pathogen was reisolated and RPB1 and RPB2 were amplified and sequenced to confirm the molecular identity of the reisolated pathogen as F. fujikuroi, satisfying Koch's postulates. To the best of our knowledge, this is the first report of Fusarium fujikuroi causing root rot of T. grandis in China.• 2023 - "First Report of Stem Rot in Cymbidium sinense Caused by Fusarium oxysporum in China", by Peng-yu Liange et al., Plant Disease.
EXCERPTS: Cymbidium sinense (Jackson ex Andr.) Willd. is a perennial terrestrial plant in the orchid family mainly in China, Japan, India, and Southeast Asia that occupies a strong position in the flower market (Zhang et al. 2013), and its roots have anti-asthmatic properties (Ke et al. 2004). In August 2020, approx. 15% stem rot on 2-year-old C. sinense with varying severity was observed in five nursery gardens in Enshi city (30°169N, 109°299E), Hubei province, China. Typical symptoms included roots and the inner part of the pseudobulbs changing from white to brown and rotting. Leaves became brown and withered from bottom to top, and there was an obvious bright yellow halo at the junction of diseased and healthy tissue, which eventually caused the whole plant to wilt and die.... Fusarium oxysporum was reisolated from the infected plants to fulfill Koch's postulates.... Members of the Fusarium oxysporum species complex are notorious for causing many diseases, including stem rot of Sulcorebutia heliosa and root rot of Torreya grandis (Garibaldi et al. 2020; Zhang et al. 2016). This is the first report of stem rot caused by F. oxysporum on C. sinense in China. The finding of this pathogen provides a clear target for stem rot control.• 2023 - "Genome Sequence and Assembly of 18 Fusarium Isolates from Florida Citrus under High Huanglongbing Disease Pressure and California Citrus under Low Huanglongbing Disease Pressure", by Tania Kurbessoian et al., Microbiology Resource Announcements.
EXCERPT: Fungal isolates were cultured from leaf, stem, and root tissue of California (Riverside County) and Florida (Marion, Lake, and Martintown Counties) citrus trees.• 2023, 2019, 2017 - Papers on Fomitiporia torreyae tree disease in China.
• Note by Torreya Guardians founder Connie Barlow:• BACK TO TABLE OF CONTENTS
From: "Species diversity of pathogenic wood-
rotting fungi (Agaricomycetes, Basidiomycota)
in China", 2023, by Yuan Yuan et al., Mycology.
Notice in the image that another genus of fungus, Fomitiporia, has the species name torreyae.
I could not track down the naming of this Chinese fungus, but presumably it was discovered in a domesticated orchard of one of the several Chinese species of genus Torreya: Torreya grandis. Notice the other tree genera it is known to infect but there is no suggestion that this newly identified fungal complex is anything other than native. See the second paper directly below. And the first paper indicates climate change is also spreading its pathogenicity.
• "First report of Fomitiporia torreyae causing trunk rot on Chinese torreya (Torreya grandis) in Anhui Province of China", 2019, by Xue Yang et al, Journal of Plant Pathology.
CONNIE BARLOW continues: "As it turns out, fungal genus Fomitiporia went through the same reanalysis about the same time as fungal genus Fusarium did in both the eastern and western hemispheres. And the result has been the same: Multiple new species complexes have been identified and named."
"The big difference: There is no suggestion that attempts must be made to ensure that the fungus does not spread beyond the tree from which it was named; its geographic range is now known to be far exceeding that of the newly named species complex."
Phase 4 - begins 2016
Fungal Endophytes Can Be Beneficial
• 2015 - "Management of Fusarium Diseases Affecting Conifers", by Thomas R. Gordon, Cassandra L. Swett, and Michael J. Wingfield, Crop Protection.EXCERPT 1: "Indications that co-occurring microbes can have suppressive effects on pathogens affecting conifer seedlings follow from the observation that although decline and death of seedlings induced by Fusarium oxysporum is common in nurseries, disease caused by this fungus is not a problem in coniferous forest soils covered with needle litter. Smith (1967) noted whereas F. oxysporum was recovered from the roots of 90% of sugar pines when trees were lifted from nursery soil, this fungus was not detectable on roots of sugar pines four years after outplanting in the forest.... F. oxysporum is rarely recovered from forest soils.... Predictions of the model were consistent with the distribution of pitch canker in North America, where the disease has been established long enough to have approached ecological limits on its geographic range."
EXCERPT 2 (absent the internal citations): "Pitch canker, caused by Fusarium circinatum, was first recognized as a disease in the state of North Carolina in the U.S. in 1945. The pathogen was isolated from cankers that were characterized by extensive production of resin, which inspired the name 'pitch canker' for the disease. Pitch canker subsequently became more widespread in the southeastern U.S. (SE U.S.), where it continues to cause problems for production of pines in plantations, seed orchards, and seedling nurseries. The disease was discovered in California in 1986, followed by confirmed reports from Japan, South Africa, Mexico, Spain, South Korea, Chile, Italy, Portugal, Colombia, and Brazil. The host range of F. circinatum extends to more than 60 species of Pinus and also includes Douglas-fir, the only conifer outside the pine genus known to be susceptible. Under experimental conditions, F. circinatum is capable of colonizing maize (Zea mays L.) asymptomatically, and natural infections of grass hosts have been confirmed in the U.S. and in South Africa. No symptoms have been observed on infected grasses, suggesting the relationship is commensal. The significance of an association with grasses has yet to be established but could influence disease development, as described below."
EXCERPT 3: Fusarium circinatum is not a common resident in soil and so is not likely to be a problem in seedling nurseries unless introduced with contaminated soil or seed. If the pathogen is not already established in soil nearby, seed is the most likely vehicle for introduction. F. circinatum has been confirmed to occur both on and within seeds of several pine species. Superficial contamination of seed is common in areas where pitch canker occurs, regardless of the disease status of the tree from which cones are obtained. Such infestations are presumably due to deposition of airborne microconidia and macroconidia, both of which F. circinatum can produce on infected host tissue. Spores might germinate on seed, but whatever subsequent growth occurs appears not to extend beyond the seed coat because topical treatments with various anti-microbial materials will eliminate the pathogen. For example, seeds collected from healthy Pinus radiata in stands affected by pitch canker commonly carry the pathogen, but the incidence of infestation can be reduced to zero by immersion of seeds in an aqueous solution of 1.0% sodium hypochlorite.... Internally infested seeds typically suffer high mortality rates from both pre- and post-emergence damping-off. However, some seed treatments will significantly reduce the rate of mortality, which indicates that internally infested seed is capable of producing a healthy seedling if growth of the pathogen can be suppressed. Both internal and external infestations of P. radiata seed were eliminated by treatment either with benomyl or a combination of benzimidazole, carboxin and thiram, which increased seedling emergence to 53 and 57%, respectively, compared to 43% for untreated seed. No seedlings emerging from treated seed were infected with F. circinatum.
EXCERPT 4: "The potential for seedlings to sustain latent infections poses a challenge for management, because visual inspections are not sufficient to identify trees that may be carrying F. circinatum. Furthermore, greenhouse studies have shown that extensive colonization of the root system by F. circinatum may not result in visible damage. Shoot symptoms become apparent only after the fungus grows into the root collar and girdles the stem. Thereafter, F. circinatum grows more extensively in roots and can be isolated from necrotic tissue. Thus it appears that rotting of roots is a consequence of seedling death and not the cause (Swett and Gordon, 2013). If this is typical of how disease develops in seedling nurseries, inspection of roots may not be a reliable means of identifying cryptic infections."
• 2016 - "Editorial special issue: Soil, Plants and Endophytes", by Stephane Compant et al., Plant and Soil.EXCERPTS: Despite the inception of the endophyte research dates back to the nineteenth century, studies on this topic have only recently bloomed, producing a fast-growing body of literature.... We currently describe endophytes as any microbe that can be isolated from asymptomatic plant tissue. This definition includes neutral, commensal and/or beneficial microorganisms as well as dormant saprobes and pathogens during their latent phase of their life cycle.• 2017 - REVIEW: "Ecology and Genomic Insights into Plant-Pathogenic and Plant-Nonpathogenic Endophytes", by Gunter Brader et al., Annual Review of Phytopathology.
... Several studies related to endophytes are, moreover, mostly biased by experimental models evaluated under gnotobiotic conditions, that is, far away from natural conditions. We need to move beyond and to analyze how the whole plant and its associated microbiota are working together under multitrophic scenarios.... SEED MICROBIONES have not been studied until recently and many aspects in the ecology of seed endophytes remain elusive. The sources of the seed microbiome, routes of transmission, possible heritability from one seed generation to the next and the role of the seed endophytes in the development of the microbiome in the emerging plant are still hardly understood.
Interestingly, the authors showed that despite differences found in soil bacterial assemblages, seed and radicle endophytic communities were similar, and that the leaf endophytic assemblage was mainly derived from the environment and not from the seed. Truyens et al. (2016) claimed that during plant growth, bacteria seem to be recruited from the environment to complement the endophytic population from which the seed endophytes of the next generation can again be selected, which is of special interest to better understand how plants select microbes eventually thriving as endophytes inside different tissues.... The effect of harsh environmental conditions on the composition of microbial assemblages has not been studied well so far.
EXCERPTS: ... Pathogenicity is a complex phenomenon. The combination of many factors, such as pathogen and host genotypes and abiotic and other environmental stresses, as well as microbial interactions, determines the outcome of the reaction of a plant to a (pathogenic) microbe. Furthermore, a single microbial species may comprise strains that are capable of exhibiting pathogenicity, mutualism, or no effect on their hosts. For instance, Fusarium oxysporum is well known for its phytopathogenic properties. Collectively, strains within the F. oxysporum species complex can cause diseases on more than 100 plant species and exhibit strong host-specificity. However, most isolates are nonpathogenic toward nonhosts, and for some isolates even biocontrol properties have been characterized.• 2018 - "Diversity and Characterization of Endophytic Fungi Isolated from the Tropical Mangrove Species, Rhizophora mucronata, and Identification of Potential Antagonists Against the Soil-Borne Fungus, Fusarium solani", by Tuan Noraida Tuan Hamzah et al., Frontiers in Microbiology.
... In this review, we address several aspects that determine the ecology and functioning of an endophyte that ultimately lead to pathogenicity or mutualism. In particular, we elaborate on how the microbial environment may influence the function of an endophyte and elucidate the differences between pathogenic and nonpathogenic endophytes, frequently mutualists, in regard to plant response, pathogen colonization behavior, and genetic variation. Overall, we postulate that plant-colonizing microorganisms, irrespective of their function as a pathogen or nonpathogen, employ shared mechanisms to interact with their plant hosts, and that pathogenicity is a consequence of fine-tuned interactions between host, environment, and other organisms.
... "The Fusarium oxysporum Complex as an Interesting Model to Understand Pathogenic and Nonpathogenic Relationships" - ... Among many fungal pathosystems, the F. oxysporum species complex represents an interesting comparative model for understanding pathogenic versus symbiotic relationships. Members within this species complex cause diseases in more than 100 plant species, but this species complex also comprises strains with biocontrol properties. One particular nonpathogenic F. oxysporum strain, Fo47, has been studied as a potential biocontrol agent of Fusarium wilt and other root diseases directly through several mechanisms, including nutrient competition, antibiosis, and mycoparasitism, or indirectly through inducing plant defense gene expression and alterations to root cell architecture.... Even though the Fo47 genome lacks the pathogenicity related SIX genes, the genome encodes for many small secreted proteins that are actively induced during the course of interactions with the plant host Arabidopsis thaliana (L.-J. Ma, unpublished data). Effectors also play significant roles in mediating endophytic and symbiotic plant-fungal interactions; however, research in this field is scarce.
EXCERPTS: The current study was designed to determine species diversity of the endophytic fungal community in the leaf tissue of R. mucronata, a tropical mangrove species commonly found at the Matang Mangrove Forest Reserve (MMFR), Perak, Malaysia, and to characterize biological activities of several selected species. These fungal endophytes might offer novel species or strains that possess valuable bioactive compounds, which demonstrates the significance of their study.... Endophytic fungi live internally within various tissues of a host plant, asymptomatically, without causing any negative effect to the host plant (Aly et al., 2011). When a host plant harbors endophytes, their concurrence may help the host to adapt to biotic and abiotic stress factors.• 2019 - "From pathogen to endophyte: an endophytic population of Verticillium dahliae evolved from a sympatric pathogenic population", by David Linnard Wheeler et al., New Phytologist
... Different parts of R. mucronata such as the bark and leaves have been used traditionally as astringent and antiseptic, and have been shown to possess activities against bacteria, ulcers, and inflammation (Kokpol et al., 1990; Suganthy and Pandima Devi, 2016).... Apart from focusing on the plant itself, several studies have been conducted on microorganisms associated with R. mucronata, including fungal endophytes. Endophytic fungi from the genera Penicillium, Ampelomyces, and Fusarium, isolated from R. mucronata, were found to be active against E. coli (Prihanto et al., 2011)....
... From the preliminary qualitative antibacterial screening of the six fungal isolates, Fusarium lateritium showed inhibition to all four bacterial pathogens tested, while Xylaria sp. to three.... Fusarium solani is a common tree pathogen and has been isolated from diseased mangrove plants (Umechuruba, 2005). We showed that endophytes Fusarium lateritium, N. oryzae, Phoma sp., and Xylaria sp., successfully inhibited the pathogenic Fusarium solani in the dual culture assay.... In contrast to Xylaria sp., F. lateritium did not display distinct scavenging activity. This result however is not as observed by previous studies, which regarded Fusarium sp. as a potent source of antioxidant based on their significant antioxidant activity recorded (Vasundhara et al., 2016). Such example include Fusarium oxysporum, an endophytic fungus from the rhizomes of Dioscorea zingiberensis (Li et al., 2012) and Fusarium proliferatum, from pigeon pea (Zhao et al., 2012b).
EXCERPTS from "Introduction": Most plants are colonized by fungi (Petrini, 1986; Wang & Qiu, 2006). These symbioses, defined by De Bary (1879) as 'the living together of dissimilar organisms', occur within the plant rhizosphere, endosphere, and phyllosphere. Regardless of the context, the outcomes of symbioses vary across a continuum, from mutualism to parasitism (Lewis, 1985). Likewise, the consequences of symbioses vary across a spectrum from terrestrialization to devastation of plants by mutualists and pathogens, respectively (Field et al., 2015; Strullu-Derrien et al., 2018). Although the consequences of symbioses are used to erect boundaries between categorical outcomes on the continuum, as well as the disciplines that study them, the fungi that interact with plants trespass these boundaries (Selosse et al., 2018).• 2020 - "Bioprospecting endophytic fungi from Fusarium genus as sources of bioactive metabolites", by Rufin Marie Kouipou Toghueo, Mycology.
... Isolates of Verticillium and Fusarium spp., for example, can be pathogens or endophytes within (Fordyce & Green, 1960; Schneider & Pendery, 1983) or among hosts (Wheeler & Johnson, 2016; Lofgren et al., 2018). For example, Fordyce & Green (1960) presented evidence that V. albo-atrum isolates were initially endophytic towards tomato and potato but, after several serial inoculations, incited symptoms on both hosts. Similarly, Wheeler & Johnson (2016) presented evidence that isolates of V. dahliae were both pathogens of potato and endophytes of several other species. Transitions between trophic states are temporally dependent (Schulz & Boyle, 2005) and governed by the physiological status of the host (Redman et al., 2001), fungus (Fordyce & Green, 1960), and/or environment (Alvarez-Loayza et al., 2011).
... At the evolutionary timescale, most endophytic representatives of the speciose fungal phylum Ascomycota arose from insect parasites (Rodriguez et al., 2008), plant pathogens (Delaye et al., 2013), lichen-forming, endolichenic, and saprotrophic fungi (Arnold et al., 2009). At the ecological timescale, some endophytes become mutualists under stressed conditions (Redman et al., 2001), while others become latent pathogens that cause disease over time (Goodwin et al., 2011), or in other hosts (Ridout & Newcombe, 2018), or become saprotrophic (Promputtha et al., 2007). The endophytic lifestyle therefore appears to be labile in most taxa studied.
EXCERPT from "Discussion" section: Endophytic Verticillium spp. have been documented since the 1960s (Fordyce & Green, 1960; Woolliams, 1966). Despite this awareness, the prevalence of these fungi in nature, their evolutionary relationships with pathogenic isolates, their role in the maintenance of inoculum, disease expression, and their contribution to population structure are unknown. This documentation is the first, to our knowledge, to describe emergence of an endophytic population of any Verticillium spp. from a pathogenic population.ABSTRACT: Endophytic fungi became an attractive source for the discovery of new leads, because of the complexity and the structural diversity of their secondary metabolites. The genus Fusarium comprising about 70 species is extremely variable in terms of genetics, biology, ecology, and consequently, secondary metabolism and have been isolated from countless plants genera from diverse habitats. These endophytic microbes may provide protection and survival strategies in their host plants with production of a repertoire of chemically diverse and structurally unprecedented secondary metabolites reported to exhibit an incredible array of biological activities including antimicrobial, anticancer, antiviral, antioxidants, antiparasitics, immunosuppressants, immunomodulatory, antithrombotic, and biocontrol ability against plant pathogens and nematodes. This review comprehensively highlights over the period 1981 to 2019, the bioactive potential of metabolites produced by endophytes from Fusarium genus.• 2020 - REVIEW: "Biological control of Fusarium wilt in crop plants using non-pathogenic isolates of Fusarium species", by Suresh Patil & S. Sriram, Indian Phytopathology.
EXCERPTS: ... Any species belonging to Fusarium genus, can be isolated as an endophyte of plants.... Undeniably, endophytic Fusarium species living inside host plant tissues without causing any symptoms of disease have proven over the years an outstanding potential by producing compounds actually approved for the treatment of several diseases including cancer, malaria, oxidative stress-related diseases, and inflammatory disorders.ABSTRACT: Many species of the genus Fusarium resemble each other morphologically. In many cases morphological differentiation is difficult, molecular tools are used. Pathogenic as well as non-pathogenic isolates have same habitat and colonize plant root system with equal measure. Since non-pathogenic isolates resemble pathogenic isolates in their nutritional and abiotic requirements, many attempts have been made to utilize them as biocontrol agents to manage Fusarium wilt diseases. Although these isolates colonize plants, they do not induce disease symptoms. They are primarily soil inhabiting in nature; once introduced in it remain in the soil for very long time providing significant and consistent disease control. Herewith we give a review and summary of different reports of non-pathogenic Fusarium in different cropping systems, mode of action (antibiosis, plant growth promotion, and induced systemic resistance), molecular basis of bioassay and identification, environmental conditions, different formulations, cross protection as well as effects on non-target crops. • 2021 - "Endophytic fungi: a tool for plant growth promotion and sustainable agriculture", by Noemi Carla Baron & Everlon Cid Rigobelo, Mycology.
EXCERPTS: INTRODUCTION: ... The taxon Fusaria is a complex one and more than one hundred formae speciales have been described. It represents rhizosphere microflora that are plant pathogens or saprophytes. Even the pathogenic isolates survive as saprophytes and once they reach the corresponding host they infect and enter into parasitic life style. Non-pathogenic isolates are capable of colonising root surfaces and protect even susceptible varieties from the highly virulent pathogenic isolates. From asymptomatic roots, avirulent Fusarium isolates can be isolated.
INDUCED SYSTEMIC RESISTANCE (ISR): Induced systemic resistance has been identified as main mode of action in many of the non-pathogenic strains of F. oxysporum. First reported by Biles and Martyn (1989) ISR has been reported by many workers....
ANTIBIOSIS: ... Hua et al. (2015) observed the non-pathogenic Fusarium isolate CanR46 could produce anti-fungal compounds 5-hexenoic acid, limonene, octanoic acid and 3,4-2H-dihydropyran that inhibited mycelium growth, germination of conidia and germ-tube elongation in Verticillium dahliae and prevented wilt in cotton.... Rodriguez et al. (2006) reported another anti-fungal compound cyclosporine. It was produced by non-pathogenic Fusarium strain S6A inhibited the sclerotia formation in Sclerotinia sclerotiorum infecting soybean. Other compounds produced by non-infective isolate include saponins, phenol, flavonoid, tannins, alkaloids, anthroquinons and terpenoids in culture filtrates and some of them are toxic to fungi and bacteria (Nawar 2016).
ROOT COLONIZATION: ... Alterations in composition of root exudates due to non-pathogenic Fusaria was observed by Schouten et al. (2009) that reduced the nematode attraction and resulted in repellence to nematodes.
PLANT GROWTH PROMOTION (PGP): Besides inducing systemic resistance, these isolates also have ability to promote plant growth as observed by Pocasangre (2000) in banana. Treatment with non-pathogenic Fusarium isolates improved the tomato plant growth parameters viz., length and weight of root and shoot (Patil et al. 2011b). All these non-pathogenic Fusarium cultures were positive for phosphate solubilisation and production of IAA and GA....
MOLECULAR CHARACTERIZATION OF NON-PATHOGENIC FUSARIUM: The sequence analysis with ITS region of the non-pathogenic Fusarium cultures along with ITS sequence of standard pathogenic isolates revealed that the Fusarium isolates viz., F. moniliforme, F. oxysporum, F. solani and F. merismoides that did not infect tomato were distinct from not only the tomato pathogen, F. oxysporum, f. sp. lycopersici, but from all the other pathogenic Fusarium of banana, chilli, red gram, bengal gram, cucurbit, etc. (Patil 2009).... Kurtz et al. (2009) used RFLP in the IGS region and found that non-pathogenic isolates of F. oxysporum f. sp. cubense were similar irrespective of their diverse geographical location.
EFFECT OF ENVIRONMENTAL CONDITIONS ON EFFICACY OF NON-PATHOGENIC FUSARIUM: Larkin and Fravel (2002) found that these isolates differed in their efficacy in different soil types. Isolate CS1 reduced wilt severity in tomato in sandy and loamy soil and not in clay soil. Isolate CS20 was effective at various environmental conditions. Non-pathogenic isolates differed in their performance at diverse temperature regime....
POSSIBILITY OF NON-PATHOGENIC ISOLATES TURNING INTO PATHOGENIC ISOLATES: Ma et al. (2010) experimentally proved that a non-pathogenic Fusarium species could become pathogenic on tomato by horizontal chromosome transfer (HCT) by transferring a pathogenicity chromosome from Fusarium oxysporum f. sp. lycopersici that causes tomato wilt. This transfer of lineage-specific chromosome between genetically distant strains indicated the polyphyletic origin of host specificity genes in Fusarium species. These lineage-specific chromosomes were rich in transposons and contained genes related to signal transduction and effector proteins related to pathogenicity and virulence. It was also hypothesized that chromosome 14 from F. oxysporum f. sp. lycopersici was responsible for pathogenicity, and transfer of this chromosome can convert the non-pathogenic strains to pathogenic to tomato. This chromosome 14 contained effector genes viz. SIX1, SIX2, SIX3, SIX5, SIX6 and SIX7, which are responsible for virulence of the pathogen.EXCERPTS: The association between plants and fungi is extremely common. Fossil records indicate the existence of this union with endophytes and mycorrhizas have existed for more than 400 million years (Krings et al. 2007; Chadha et al. 2015) starting when plants colo- nised the soil, thus indicating the importance of this group in the evolution of this process (Rodriguez et al. 2009; Rai et al. 2014; Anjum et al. 2019). The positive aspects of this interaction have always been noted and discussed, but in-depth studies evaluating the real benefit provided by these fungi have only been performed recently (Busby et al. 2016; Card et al. 2016; Vega 2018; Quesada-Moraga 2020).• 2021 - "Protection to Tomato Wilt Disease Conferred by the Nonpathogen Fusarium oxysporum Fo47 is More Effective Than that Conferred by Avirulent Strains", by Francisco J. de Lamo et al., Phytopathology.
... The endophytic interaction is defined as balanced antagonism (Schulz et al. 2015) because the recognition of the plant as a host requires the activation of virulence mechanisms for colonisation and the triggering of host defences by these events. While an equilibrium exists in this interaction, the fungus survives of nutrients from the host plant and, in exchange, provides benefits, including tolerance to biotic and abiotic stresses (Bamisile et al. 2018).... Fungi are able to act as antagonists of plant pathogens through the use of a diverse range of mechanisms, such as the production of metabolites (antibiotics, volatile compounds and enzymes), engagement in competition (for space, carbon sources, nitrogen and minerals) and parasitism, induction of systemic resistance by the plant and increases in plant growth, resulting in the reduction of the activity of the pathogens (Vega et al. 2009; Vidal and Jaber 2015; Vega 2018; Lr 2018; Quesada- Moraga 2020).
... Some fungal endophytes are able to colonise a wide range of plant species, while others are more specific and occur only inside a restricted number of plants. Additionally, specificity can also be present in relation to the portion of the plant that is colonised (Aly et al. 2011; Bamisile et al. 2018). Apparently, vertically transmitted fungi seem to present plant associations with a more mutualistic profile than horizontally transmitted fungi, which are more likely antagonists (Aly et al. 2011).... Horizontal transmission occurs when vegetative propagules or spores are produced by the endophyte and spread to the plant population through the air or via some vector, while vertical transmission consists of the transference of the fungi to the plant progeny via seeds.
... The genetic mechanisms involved in stress tolerance are poorly known, and an essential aspect is not considered in this process: the symbiotic association of plants and microorganisms (Chadha et al. 2015). The fungal endophyte community that exists in wild plants can be severely modified, and many representatives can be lost during domestication; therefore, fungi are harmed by losing their safe niche and plants are deprived of a partnership that could improve their ability to overcome environmental challenges (Lugtenberg et al. 2016). For a deeper discussion about the reasons why endophytes can be lost during plant breeding, see Lugtenberg et al. (2016). For endophytes, the inner part of the plant is a protected niche that contains the necessary nutrients for fungal survival and growth in addition to presenting low competition with other microorganisms. Therefore, in exchange for this safe place, fungi improve plant fitness by several mechanisms (Khan et al. 2015; Lugtenberg et al. 2016; Chitnis et al. 2020). The benefits of plant colonisation by endophytic fungi can occur directly and/or indirectly, and the differentiation among them is complex (Berg 2009). Among the direct mechanisms of growth promotion, the most important are the acquisition of nutrients and the production of phytohormones, while tolerance to biotic and abiotic stresses, including combat against pathogens, is considered an indirect aspect in the promotion of growth (Hardoim et al. 2015; Souza and dos Santos 2017).
Production of phytohormones: Endophytic fungi are able to produce auxins, gibberellins (GAs) and cytokinins. The potential of phytohormone production by endophytic fungi is underexplored, especially for gibberellins, even though these molecules are as important as chemical signalling and messengers for plant growth in different environmental conditions (Khan et al. 2015).... Gibberellins are essential in several plant responses, including seed germination, stem elongation, sexual expression, flourishing, fruit formation and senescence (Bomke and Tudzynski 2009). The production of gibberellins by endophytic fungi is described as occurring from acetyl-CoA by the mevalonic acid (MVA) pathway....
Activation of systemic resistance: Endophytic fungi can aid plants in improving their self-defence system, thus promoting the activation of induced systemic resistance (ISR) pathways, which may overlap with that of acquired systemic resistance (ASR) because both systems can improve plant growth (Berg 2009; Busby et al. 2016) and protect against pests and pathogens (Chadha et al. 2015).... The balanced interaction between fungal endophytes and their plant hosts occurs due to the lack of pathogenic properties. A good example is the comparison of Brassicaceae's endophytic strain Colletotrichum tofieldiae and the pathogenic Colletotrichum incanum in Arabidopsis thaliana. Evolution has negatively selected genes of effector proteins in the endophytic strain, which are directly involved in the pathogenic action at the moment of plant colonisation, and the same did not occur with the pathogenic strain C. incanum.
Production of antibiotics and secondary metabolites: In addition to stimulating the production of defence molecules by the plant itself, endophytic fungi are a large reservoir of molecules that act in favour of their host. They are excellent producers of compounds with activity against pathogens and herbivores, including alkaloids, steroids, terpenoids, peptides, polyketones, flavonoids, quinols, phenols, chlorinated compounds and volatile organic compounds (VOCs) (Card et al. 2016; Lugtenberg et al. 2016; Latz et al. 2018; Kaddes et al. 2019). Moreover, studies report the production of compounds with antiviral, antibacterial, antifungal and insect action (Card et al. 2016; Latz et al. 2018).... An uncountable number of molecules are produced as secondary metabolites by endophytic fungi; however, specific pathways and substances have not been well characterised thus far. The review of Lugtenberg et al. (2016) is recommended for deeper knowledge of the chemical structures of some secondary metabolites produced by endophytic fungi.... Among the wide range of secondary metabolites produced by endophytic fungi, more than 300 of these molecules are VOCs (Lugtenberg et al. 2016; Kaddes et al. 2019). They consist of small molecules, presenting high vapour pressure, and they are easily diffusible through the cell membrane, in the atmosphere and in the soil, which makes them special agents of fungal communication with other organisms, including plants, in addition to presenting bioactivity against many pathogens (Kaddes et al. 2019).
Protection against biotic and abiotic stresses: Environmental degradation by agricultural processes and global climate change expose plants to increasingly challenging conditions for their growth and maintenance.... Endophytic fungi are able to combat abiotic stresses, including drought, high and low temperatures, salinity and toxic heavy metals (Aly et al. 2011; Khan et al. 2015). For biotic stress protection, fungal endophytes are responsible for the activation of ISR and ASR, which produce metabolites against pathogens; moreover, parasitism or competition can occur to avoid disease and herbivory (Chadha et al. 2015; Chitnis et al. 2020).... Endophytic fungi help host plants adapt to stress conditions through diverse mechanisms. As reviewed by Khan et al. (2015) and Yan et al. (2019), during oxidative stress, plants increase the activity of antioxidant enzymes, mainly catalases and peroxidases, which leads to the production of ROS, resulting in membrane attack causing the peroxidation of membrane lipids. By some not yet defined mechanisms, endophytic fungi confer tolerance to ROS, reducing lipid peroxidation. Another important problem caused by abiotic stresses (drought, heat and salinity) in membranes is electrolyte leakage, which is associated with the variation in the lipidic composition and the amount of these molecules of the cell membrane due to stress conditions. Endophytic fungi are able to induce changes in the lipidic composition of the cell membrane, preventing leakage (Khan et al. 2015; Yan et al. 2019).
... In relation to biotic stress, the main defences against pathogens, herbivores and nematodes are the production of secondary metabolites and the activation of systemic resistance by endophytic fungi (Latz et al. 2018; Yan et al. 2019; Poveda et al. 2020).ABSTRACT EXCERPT: Although the vascular pathogen Fusarium oxysporum is notorious for being the causal agent of Fusarium wilt disease, the vast majority of F. oxysporum strains are harmless soil and root colonizers. The latter F. oxysporum's are often endophytes colonizing roots intracellularly without negatively affecting plant fitness. Actually, most of them, like Fo47, are beneficial providing biological control to various root pathogens.
EXCERPTS: Fusarium oxysporum, a filamentous ascomycete, is the causal agent of Fusarium wilt disease (Michielse and Rep 2009). This soilborne vascular fungus ranks among the top 10 major fungal plant pathogens (Dean et al. 2012). Each F. oxysporum pathogen harbors a specific set of effector genes required for pathogenicity on a specific host (van Dam et al. 2016). Nevertheless, the ample majority of F. oxysporum strains are nonpathogenic saprotrophs able to endophytically colonize plant roots (Bao et al. 2004). It has been well established that wilt disease-suppressive soils contain beneficial F. oxysporum endophytes that are responsible for protecting a susceptible host to pathogenic F. oxysporum strains (Alabouvette 1986; Tamietti et al. 1993). These F. oxysporum endophytes exert biological control by directly affecting the invading pathogen and/or by inducing plant immune responses.... In summary, compared with the endophyte Fo47, avirulent pathogens only weakly reduce susceptibility to F. oxysporum f. sp. lycopersici. A possible explanation for this observation is that the Fo47 endophyte is less, or even unable to suppress host immune responses. Presumably, once immune responses are triggered by an endophyte they cannot be suppressed by the effectors present in a virulent strain, or it is to a much lower extent not able to significantly contribute to resistance.
• 2022 - "Apple Endophytic fungi and their antagonism against apple scab disease", by Leila Ebrahimi et al., Frontiers in Microbiology.EXCERPT: Endophytic fungi are microorganisms with the ability to colonize plants for the entire or at least a significant part of their life cycle asymptomatically, establishing a plant-fungus association. They play an important role in balancing ecosystems, as well as benefiting host through increasing plant growth, and protecting the host plants from abiotic and biotic stresses using various strategies. In the present study, endophytic fungi were isolated from wild and endemic apple cultivars, followed by characterizing their antifungal effect against Venturia inaequalis.• 2023 - "Acuminatums E and F, two new cyclic lipopeptides from Fusarium lateritium HU0053 and their antifungal activity", by Zhi-Juan Zhong et al., Journal of Asian Natural Products Research.
ABSTRACT: Two new cyclic lipopeptides, acuminatums E (1) and F (2), together with four known cyclic lipopeptides, acuminatums A - D (3 - 6) were isolated from the corn culture of endophytic Fusarium lateritium HU0053. Their structures were elucidated by spectroscopic and advanced Marfey's amino acid analysis. All compounds were found to exhibit antifungal activities against Penicillium digitatum. Acuminatum F (2), a new cyclic lipopeptide containing an unusual 3, 4-dihydroxy-phenylalanine unit exhibited the strongest antifungal activities with inhibition zone of 6.5 mm at the dose of 6.25 ug. Therefore, acuminatum F might be a potential environmental-friendly preservative for citrus fruits.• 2023 - REVIEW: "The endophytic Fusarium strains: A treasure trove of natural products", by Arwa Mortada Ahmed et al., Royal Society of Chemistry.
EXCERPTS from "Introduction": ... Fusarium is considered as a member of the most dominant endophytic fungal genera in the world, characterized genetically with extraordinary discrepancy, together with its ability to grow on a wide range of substrates and their efficient mechanisms for dispersal, which affect their biology and interaction with their surrounding organisms, together with secondary metabolism that makes Fusarium an important group of fungi.... Fusarium is considered to be a rich source of bioactive compounds, including more than one hundred compounds with unique chemical structures, among more than three hundred compounds of various classes such as butenolides, alkaloids, terpenoids, cytochalasins, phenalenones, xanthones, sterols, and diphenyl ether and anthraquinone derivatives, with multidimensional bioactivities such as antimicrobial, antiviral, anticancer, antioxidant, antiparasitic and immunomodulating activity.... This review covers the literature on the isolated natural products from the fungal endophyte Fusarium species and their various bioactivities from April 1999 to April 2022.• 2019 - REVIEW: "Bacterial and Fungal Endophytes: Tiny Giants with Immense Beneficial Potential for Plant Growth and Sustainable Agricultural Productivity", by Olawale Israel Omomowo and Olubukola Oluranti Babalola, Microorganisms.
EXCERPTS from "Conclusion and future perspectives": Fungal endophytes are the fungal population of the internal tissues of plants causing no apparent symptoms of disease. These endophytes are able to produce certain phytoconstituents originally attributed to their host plant, which is thought to be due to genetic recombination between the endophyte and the host plant that occurred in the evolutionary period. In addition, fungal endophytes are reported to produce new secondary metabolites, which may be totally different from those of the host plant, so they are a precious source of novel bioactive natural products and an alternative source for phytochemicals initially produced by higher plants. The Fusarium genus is among the well-known genera of fungal endophytes with many different species. Fusarium endophytes are considered to be a rich source of new secondary metabolites with valuable biological activities and a promising basis for drug discovery.... Fusarium species have the ability to produce a varied array of secondary metabolites such as sterols, polyketides, alkaloids, terpenes, peptides and other compounds.
EXCERPTS: Endophytes are a group of ubiquitous and diverse microbes that are found in diverse ecological niches in the tissues of a plant. They inhabit all plants asymptomatically and serve as a treasure trove of biologically important metabolites that could be used for promoting plant growth, potent biocontrol against pathogens and pests, immune defense and fitness, and for conferring functional traits on the plant to allow it to withstand or tolerate external stresses.• 2021 - "An Ecological Insight into the Multifaceted World of Plant-Endophyte Association", by Sushma Mishra et al., Critical Reviews in Plant Sciences.
...All living plants are colonized by different endophytes, without any exceptions; indeed, plants are host to a diverse group of endophytic microbes in a mutualistic, beneficial way, which is vital to plant growth and health. Endophytic microbes can enter and colonize plants through the vertical seeding method or through horizontal transmission from the soil to the plants. Either way, mutualistic benefits are still conferred through the plant-microbe interactions. This highly beneficial mutualistic interaction makes it possible for endophytic microbes to confer the following benefits on their plant host: they increase the nutrients made available to the plant host, act as protective defenders against pathogens and destructive pests, improve the capability of the plant to withstand or tolerate environmental stresses, help in modulating development of their plant host, and are also helpful in tackling issues related to the growth of unwanted weeds.
... Fungal endophytes exist in close, mutually beneficial association with their plant host, in that they provide ecological support to their host plants by allowing them to survive adverse biotic and abiotic stresses. In their turn, the endophytes derive nutrients and protection from the plant. Fungal endophytes colonize plant host tissues such as stems, fruits, flowers, roots, leaves and branches; this is done asymptomatically, without any adverse effects. These include the containment or mitigation of damage caused by pests or destructive insects. Moreover, reports indicate that plants colonized by these fungal endophytes are less susceptible to the destructive effects of pests. The endophytes confer these benefits on their host plant by interrupting the growth and development phases of the pest; they also affect the feeding pattern and reproductive stages of the pest, thereby affecting its overall survival.... The lessening of damage through the containment of pests by the fungal endophytes can be attributed to their production of secondary mycotoxigenic metabolites in their host plants which are toxic to the pests.
Another important mutualistic benefit of the endophytic fungal interrelationship with the plant host is the ability of the endophyte to confer both abiotic- and biotic-stress-tolerant traits on its plant host, thereby helping the host to improve its growth and to be less susceptible to disease. Fungal endophytes have also proven to be important sources of metabolically active compounds. They have the ability to produce vital plant hormonal compounds such as piperine, gibberellic acid, and indole-3-acetic acid, which are required for the promotion of plant growth. They are capable of inhibiting plant disease causing pathogens, and enable plants to tolerate salinity stress, among other stresses.
It can be asserted based on studies done in the last few years that endophytic fungi play key functional roles in the ecosystem. Their main effects in terms of beneficial interactions with plants include their ability to make vital growth-promoting nutrients available to the plant; they help suppress noxious plant pests, pathogens, nematodes, and other destructive insects; they help in mitigating environmental stress; and they are also useful in the bioremediation of environmental contaminants. They are able to achieve these effects by using different mechanisms.
... Endophytic fungi positively influence exudations in the roots of plants, thereby attracting beneficial rhizosphere microbiota that facilitate the transportation of minerals from the soil that are required by plants. They are also beneficial partners in promoting plant immunity and in ecological and physiological adaptations in their plant hosts, thus enabling them to adapt to environmental stresses and to fight against pathogens.
... Endophytic bacteria and fungi that are associated with the colonization of plant tissues are transmitted horizontally (plant or soil to plant), vertically (parent plant to seed), or in a mixed way. MOST FUNGAL ENDOPHYTES ARE VERTICALLY TRANSMITTED THROUGH THE SEED. Endophytic bacteria, on the other hand, prefers horizontal transmission. Bacterial endophytes are optimally adapted to the horizontal transmission route, although there have been reports of vertically transmitted seed-borne bacterial endophytes that are of biotechnological importance.EXCERPTS: ... Based on the phylogenetic affiliation, metabolic potential, physiological features, and mode of transmission, endophytes can be classified into systemic and nonsystemic endophytes (Wani et al., 2015). Systemic endophytes, also known as true endophytes, represent the typical class of endophytes that exist in a symbiotic relationship with the host plant, and do not cause disease at any stage of the life cycle. The systemic endophytes are also co-cladogenetic, i.e. the plant may host the same set of endophytes irrespective of environmental conditions (Botella and Diez, 2011; Higgins et al., 2014). Nonsystemic endophytes, on the other hand, are the microorganisms that live asymptomatically within the plant tissues for at least a part of their life cycle. Such associations are typically short-lived and seasonal; their diversity varies with changes in the host's physiological parameters and varying environmental conditions (Botella and Diez, 2011). Another striking feature of nonsystemic endophytes is that they may turn pathogenic when the host plant is stressed or resource-limited (Petrini, 1991).
A plant may attain its microbiota either through horizontal transmission, i.e. from the environment (mainly rhizosphere or phyllosphere), and/or vertical transmission, i.e. from one generation to the next, through seeds or vegetative propagules (Frank et al., 2017; Kumar et al., 2020).
In another study, the same microbe Fusarium verticillioides, demonstrated two modes of lifestyle in maize: pathogenic and endophytic, depending upon the environmental factors and the host genotype (Bacon et al., 2008). The authors reported that the fungus switches to its pathogenic state when the plant is stressed.... It needs to be emphasized that the boundaries between the three types of symbiotic associations (mutualism, commensalism, and parasitism) are not well demarcated. Depending on many factors including environmental conditions, biotic and abiotic stresses, and microbial interactions, the equation between the plant and microbe changes.
Phase 5 - begins 2018
Fusarium torreyae in "all seeds"
Confirms Core Mutualism
• 2018 - "What Is There in Seeds? Vertically Transmitted Endophytic Resources for Sustainable Improvement in Plant Growth", by Raheem Shahzad et al., Frontiers in Plant Science.EXCERPTS: [This paper is a "Mini-Review"; Fusarium genus is mentioned several times, both as pathogenic and as mutualistic.]• 2019 - REVIEW: "The seed microbiome: Origins, interactions, and impacts", by Eric B. Nelson, Plant and Soil
Seeds are not merely the carriers of a plant's hereditary information, but also both reservoirs for plant microbiota and vehicles for their vertical transmission. The role of seed-associated microbes is of significance to plant growth and development because these microbial communities may secrete important phytohormones, such as cytokinins, that break seed dormancy and inhibit microbial invasions.
Recent evaluations suggest that over 300,000 plant species are found worldwide, and that every plant carries at least one endophyte. Indeed, endophytic microbes have been found in every plant species examined to date.... It can be assumed that plants deprived of endophytes would be more vulnerable to environmental stress and pathogenic attacks. Endophytic microorganisms (bacteria or fungi) are a key class of plant symbionts that live inside plant tissues without inducing any disease symptoms, and which are associated with the plant throughout its life history, from seed germination to fruit development. Endophytes are found in the roots (rhizosphere), leaves (phylloplane), stems (laimosphere and caulosphere), fruits (carposphere), seeds (spermosphere), and flowers (anthosphere), as described by many scientists.
... The relationship between endophytes and plants is unique in the ability of the former to provide alternative sources of biologically active metabolites, such as enzymes, biofunctional chemicals, phytohormones, nutrients, and minerals, and to facilitate the distribution or production of these resources which contributed in the elimination of various stresses. In return, the host plant provides a protective sanctuary for the microbes within the plant tissues, in which they can grow and reproduce, but without compromising the plant's own growth resources.
... Seed-borne endophytes are of particular importance because they are passed between successive plant generations via vertical transmission, thus ensuring their presence in the next generation of seedlings.... The internal environment of a seed changes during maturation, which consequently affects the seed endophytic community. The ability to reside in a seed and adapt to severe environmental conditions are special characteristics of seed endophytes that are rarely found in endophytes isolated from roots, shoots, or other plant tissues. Seed endophytes have the ability to form endospores, thus providing protection from changing conditions inside the seed. They also maintain other features, such as cell motility and phytase activity, in order to be able to migrate freely inside the plant and enter the seeds before they harden. There have however been relatively few studies examining biodiversity in seed-borne endophytes.... Conservation of vertically transmitted endophytes indicates an evolved form of mutualism.... Some studies indeed report that the rate of vertical transmission for many fungal endophytes is greater than 90%.BACKGROUND: The development and dispersal of seeds as well as their transition to seedlings represent perhaps the most critical stages of a plant's life cycle. The endophytic and epiphytic microbial interactions that take place in, on, and around seeds during these stages of the plant's life cycle may have profound impacts on plant ecology, health, and productivity. While our understanding of the seed microbiota has lagged far behind that of the rhizosphere and phyllosphere, many advances are now being made.• 2020 - "Extending Plant Defense Theory to Seeds", by James W. Dalling et al., Annual Review of Ecology, Evolution, and Systematics.
EXCERPTS: In discussing seed microbiomes, it is important to distinguish between the endophytic microbiota (i.e., those microbial species that reside in internal seed tissues and vertically-transmitted to progeny seedlings) and epiphytic microbiota (i.e., those microbial species that colonize seed surfaces and may or may not become internalized within seed tissues and transmitted either vertically or horizontally). Although this is a rather artificial division, in part because endophytes can become epiphytes and vice versa, the reasoning for distinguishing them is that the endophytic microbiota may often originate from different seed tissues or environmental sources than those of the epiphytic microbiota. For example, microbes associated with the embryo and endosperm are more likely to be transmitted vertically than those associated with the seed coat, which are likely to be much more diverse and transmitted horizontally (Barret et al. 2016).
... For many years, the endophytic seed microbiota has been viewed as being composed of taxa that are strict commensals or mutualists (Hume et al. 2016; Malfanova et al. 2013; Muller et al. 2016; Porras-Alfaro and Bayman 2011; Santoyo et al. 2016; Truyens et al. 2015), despite our recognition of endophytic viral, bacterial, and fungal seed-borne pathogens as early as the 1940s (Munkvold 2009). The endophytic lifestyle does not necessarily imply ecology or a functional characterization of plant responses to the presence of the microbe (Schulz and Boyle 2005) and we now clearly recognize that mutualism and pathogenicity are not inherent microbial properties and are only expressed within certain contexts (Alvarez-Loayza et al. 2011; Eaton et al. 2011; Fesel and Zuccaro 2016; Malcolm et al. 2013). I believe this contemporary view better advances our understanding of the ecology of plant-microbe interactions in general but of seed-microbe interactions specifically. Therefore, for the purposes of this review, I will adopt this latter view.
... The other ascomycete classes contain the commonly described endophytic genera Chaetomium, Fusarium (and associated teliomorphs), Microdochium, Stemphylium, and Xylaria. In addition to being present in seeds, many of these fungal genera are commonly associated with soils where they are frequently transmitted horizontally to plants. In fact, recent studies have demonstrated that local site conditions and not host genotype may have a strong influence on the assembly of fungal seed microbiomes (Klaedtke et al. 2016). Although in many cases the physical location of fungi in the seed is not clear, they may largely reside on and in the seed coat (Rodriguez et al. 2009) where they may be both vertically and horizontally transmitted to subsequent generations.
... Seed germination and seedling growth is the stage of the life cycle where all the previous interactions with microbes potentially have their ultimate impact. While microbial interactions during flowering, seed dispersal, and dormancy in the seed bank may all impact the microbial legacy of a seed, the interactions of seed-associated microbes with the soil microbiota during seed germination may have the strongest impacts on overall plant fitness. Both the endophytic and epiphytic seed microbiota as well as the soil microbiota are activated during seed imbibition and germination to create a spermosphere environment either beneficial or detrimental to the critical seed-to-seedling phase of the life cycle. Whether the origin of the spermosphere and seedling microbiota arises from the endophytic seed microbiome, the soil microbiome, or from a combination of both remains unresolved. However, recent studies are revealing new insights into this question and improving our understanding of how these interactions may ultimately influence plant health management in agriculture and ecological dynamics in natural ecosystems.
... While it should be clear that seeds associate with a large diversity of both endophytic and epiphytic microbes, the connections between the two and their interconnectedness with the soil are just beginning to be realized. Research exploring the movement of seed endophytic microbes into the spermosphere and rhizosphere as well as recruitment from the spermosphere and rhizosphere and into the endophytic seed microbiome offer exciting possibilities for better understanding the dynamics of plant microbiomes.
EXCERPTS: ... We also highlight recent insights into the mutualistic and antagonistic interactions between seeds and microbial communities, including fungi and endohyphal bacteria, that can influence seed survival in the soil and subsequent seedling vigor.• 2021 - "Seed-Transmitted Bacteria and Fungi Dominate Juvenile Plant Microbiomes", by David Johnston-Monje et al., Frontiers in Microbiology.
Seed bank persistence. Seeds that persist for prolonged periods before germination, and consequently become incorporated in the soil seed bank, may have a reduced risk of predation (Hulme 1998). However, once in soil they are increasingly susceptible to microbial infection from pathogens and decay of seed-protecting structures by saprophytes (Wagner & Mitschunas 2008). Seed bank-forming species might therefore be expected to undergo selection for particularly effective antimicrobial defenses relative to species with a more transient seed stage. Effective defenses for persistent seeds may include indirect defenses via microbial mutualistic partnerships that exclude potential pathogens or promote germination (Sarmiento et al. 2017) or the induction of biochemical defenses consisting of latent enzymes that are activated in the presence of pathogens (Fuerst et al. 2017).
... Within this framework, species with physical dormancy use physical defenses to exclude predators and germinate rapidly to evade pathogens. Species with physiological dormancy are de- fended with varying investments in chemical and physical defenses. Quiescent seeds, which have comparatively limited chemical and physical defenses, may instead depend on beneficial seed-infecting microbes for protection.EXCERPTS: Over hundreds of millions of years, angiosperms have coevolved with microbes that helped them acquire nutrients, resist stress, and combat pathogens. Today, plants are considered to be holobionts, a community of microbes cooperating and coevolving with their host to stimulate its anatomy, physiology, development, immunity, behavior, and genetic variation.... It was not until the advent of high-throughput sequencing technologies at the beginning of this new millennium, however, that the immense diversity of plant-associated microbes began to be understood by the broader scientific community, highlighting the potential to discover many new beneficial plant-associated bacteria and fungi.... If plants are truly holobionts that have survived and coevolved with microbes for hundreds of millions of years (Vandenkoornhuyse et al., 2015), it makes sense that their most important symbionts would be vertically transmitted through seed rather than gambling that all of the correct soil-dwelling microbes might be available at the germination site (Nelson, 2018).... Although much work needs to be done to better understand the importance of seed endophytes, it has been shown that they can aid in germination, provide protection from pathogens, and improve mineral nutrition and vigor of the seedling.... Seeds and the area around the germinating seed (spermospheres) are increasingly being appreciated as microbial habitats contributing microbiota that can protect seeds against rotting in the soil, aid in germination, and increase seedling vigor (Nelson, 2018).• 2021 - "Experimental Evidence of Microbial Inheritance in Plants and Transmission Routes from Seed to Phyllosphere and Root", by Ahmed Abdelfattah et al., Applied Microbiology International.
... It is troubling to think that because of the use of vegetative propagation in plants like cassava, potatoes, and strawberries, in addition to the phytosanitary standards requiring the physical and chemical disinfestation of botanical or vegetative seeds in order to have pathogen-free crops, the normal transmission of microbes from seeds to seedlings may have been interrupted by modern agriculture. Regardless of provenance, with thousands of different species of microbe in the plant biome, how does one determine which are the most important to the plant's well-being and productivity?
... When trying to identify a core microbiome, occupancy (how often a microbe is observed in a sample) is most often considered the defining characteristic. For example, one definition of a core microbiome are those bacteria and fungi that are closely associated with a particular species or genotype of plant (i.e., high occupancy), independent of environmental conditions. Core microbiomes are thought to contain key microbial taxa that have been important for plant survival and reproduction over evolutionary time. Such microbes, must have over millions of years, developed a robust and efficient transmission strategy and retained the ability to colonize the plants and also to provide beneficial functions that contribute to plant growth, survival, and/or reproduction; traits which could be under positive selection in the holobiont.
... The primary purpose of this study was to demonstrate the importance of seed transmission to the establishment of plant microbiomes, which have traditionally been assumed to acquire all their microbes from soil. Showing that seeds are dominant players in establishing plant microbiomes could lead to a paradigm shift in our understanding (and ability to manipulate) of plant microbiome assembly, which has been assumed to depend largely on soil. A secondary purpose of this study was to establish whether core seed-transmitted microbiomes might exist across these economically important plant species.... Our experiment attempts to document the common (appearing in over 60% of samples) and core (appearing in 100% of samples) microbes inhabiting seed interiors and seed surfaces (spermospheres) of a panel of 17 academically and economically important plant species, many of which have had their genomes sequenced and serve as model organisms.... In seeds or spermospheres, only Pantoea (BactOTU1), Enterobacter (BactOTU2), Pseudomonas (BactOTU3), Bacillus (BactOTU8), and Fusarium (FungOTU1) appeared to be part of a core microbiome across the plant species.... Seed-borne Fusarium has been identified as one of the dominant members of the stem endosphere mycobiome of maize.... In our experiment, the dominant genera of seed-transmitted fungi in roots grown on sterile sand were Fusarium, Alternaria, Pseudozyma, Sarocladium, Penicillium, and Phoma, which also dominated soil-grown roots, although only Fusarium and Pseudozyma occurred in all samples. Both Fusarium and Alternaria have been identified as core root fungi in comparisons of poplar, oak, and pine.
SUMMARY EXCERPTS: While the environment is considered the primary origin of the plant microbiome, the potential role of seeds as a source of transmitting microorganisms has not received much attention. Here we tested the hypothesis that the plant microbiome is partially inherited through vertical transmission. An experimental culturing device was constructed to grow oak seedlings in a microbe-free environment while keeping belowground and aboveground tissues separated. The microbial communities associated with the acorn's embryo and pericarp and the developing seeding's phyllosphere and root systems were analysed using amplicon sequencing of fungal ITS and bacterial 16S rDNA. Results showed that the seed microbiome is diverse and nonrandomly distributed within an acorn. The microbial composition of the phyllosphere was diverse and strongly resembled the composition found in the embryo, whereas the roots and pericarp each had a less diverse and distinct microbial community. Our findings demonstrate a high level of microbial diversity and spatial partitioning of the fungal and bacterial community within both seed and seedling, indicating inheritance, niche differentiation and divergent transmission routes for the establishment of root and phyllosphere communities.• 2022 - OPINION: "From seed to seed: the role of microbial inheritance in the assembly of the plant microbiome", by Ahmed Abdelfattah et al., Trends in Microbiology.
INTRODUCTION EXCERPTS: All plant species, without exception, have been found to be associated with a diverse community of microbes. The first known association between two microorganisms occurred around 1.45 billion years ago, between a prokaryote now known as a mitochondrion and an archaeon, which led to the creation of eukaryotes (Roberts, 2017). A billion years later, the merging of a eukaryote with a photosynthetic bacterium (now known as a plastid) represented the first step of plant evolution (Sagan, 1967; Gray, 2017). In current ecological systems, plants have been shown to recruit, actively or passively, microbes that form a dynamic assemblage of species referred to as the plant microbiome. The fossil record indicates that plants have been associated with endophytic fungi and bacteria for more than 400 million years (Krings et al., 2007). Plants pass vital endosymbionts such as mitochondria and chloroplasts to their offspring through cytoplasmic inheritance; however, data on the inheritance and continuity of a distinct plant microbiome, and how the transfer occurs, are lacking. This raises a fundamental question, where do plant-associated microbes come from?
DISCUSSION EXCERPTS: ... The fact that the transmitted microbiome represented a very large fraction of the seed microbiome further emphasizes the ecological role of seeds as a reservoir and source for community assembly in new seedlings (Shade et al., 2017).... Although the fungal and bacterial community in the phyllosphere reflected the community present in the embryo, only part of the microbes present in the embryo and pericarp were identified in the roots of the developing seedling. As a result, the microbial community of the phyllosphere differed significantly from the root community, despite originating from the same source. These differences could be due to a combination of microbial life-history traits and plant regulatory factors that limit the migration of some members of the seed microbiome into the roots. Whether direct or indirect, plant-mediated or microbe-mediated, results of the present study indicate that the distinct microbial communities commonly reported between above- and below-ground plant parts in natural and agricultural studies may originate during the transmission of microbes from seed to seedling, and not, as commonly assumed, due to inherent differences in the microbial communities in the soil and air. Such initial seed-borne differences among the below- and above- ground plant tissues may cause strong priority effects during later development. Our demonstration of partial transmission of the seed microbiome to the roots confirms previous work suggesting that plant seeds are a repository for rhizosphere microbial communities (Kong et al., 2019), and the high resemblance of the embryonic and phyllosphere microbiome indicates that seeds may play an even larger role as a repository for the phyllosphere microbial community.
... We chose to focus on the core microbiome in an attempt to identify those taxa that have a consistent presence in the acorn, and that were consistently transmitted to the developing seedling. Such core taxa are expected to play a significant functional role in the respective compartments of the holobiont (Vandenkoornhuyse et al., 2015). In this regard, several taxa were found to be vertically transmitted include taxa with various functions such as the production of antimicrobial compounds, detoxification, nutrient uptake and growth-promoting activities.... The collective data provide clear evidence of vertical transmission from seed to seedling and highlight the important role of vertical transmission during the assembly of the plant microbiome.
EXCERPTS: Despite evidence that the microbiome extends host genetic and phenotypic traits, information on how the microbiome is transmitted and maintained across generations remains fragmented. For seed-bearing plants, seeds harbor a distinct microbiome and play a unique role by linking one generation to the next. Studies on microbial inheritance, a process we suggest including both vertical transmission and the subsequent migration of seed microorganisms to the new plant, thus become essential for our understanding of host evolutionary potential and host-microbiome coevolution. We propose dividing the inheritance process into three stages: (i) plant to seed, (ii) seed dormancy, and (iii) seed to seedling. We discuss the factors affecting the assembly of the microbiome during the three stages, highlight future research directions, and emphasize the implications of microbial inheritance for fundamental science and society.• 2023 - Insights into the seed microbiome and its ecological significance in plant life", by Aadil Farooq War et al., Microbiological Research.
... Spermatophytes (seed plants) have evolved as an adaptation to the terrestrial lifestyle during the late Devonian period. Seeds served as the plant's means of survival during harsh periods, facilitated dispersal across the landscape, and provided plant embryos with the protection and nourishment necessary to withstand external disturbances, especially desiccation. During the evolution of seed plants, plants formed symbiotic relationships with microorganisms, which facilitated their establishment on land and subsequent evolutionary divergence. Seeds themselves contain diverse microbial communities, which are present within their tissues (seed endophytes) and on their surface (seed epiphytes). The existence and function of the seed microbiome, however, has remained unnoticed for decades. The first suggestions regarding the presence of microorganisms in seeds came from studies that focused on pathogenic fungi, giving the impression that seeds free of microorganisms reflected higher seed quality. This gave rise to the widespread use of physical and chemical treatments of crop seeds.
... Early evidence that seeds harbor beneficial microorganisms was described in a study on the symbiosis between endophytic fungi of the family Clavicipitaceae and tall fescue Festuca arundinacea [1994 and 2004]. These findings were pivotal in changing our view on seed-borne microorganisms and altered the notion that seed sterilization processes would necessarily result in a healthier crop. Since then, seed microbial communities including fungi, bacteria, and archaea have been repeatedly shown to be a reservoir of specialized microorganisms. Currently it is evident that seeds contain mainly plant-beneficial microorganisms many of which are involved, directly or indirectly, in fundamental physiological processes, such as seed dormancy and germination, environmental adaptation, resistance and tolerance against diseases, and growth promotion. Yet, compared to other plant compartments, such as the phyllosphere and rhizosphere, studies on the seed microbiome are lagging.
... Despite the variation in species richness among seeds belonging to different species, some microbial groups are commonly found in seeds, and are collectively referred to as the seed core microbiome. These include members of the orders Capnodiales and Pleosporales for fungi, Pseudomonadales and Enterobacteriales for bacteria, as well as very low abundances of the phyla Thaumarchaeota and Euryarchaeida for archaea. In a meta-study on 50 plant species, Cladosporium perangustum, Alternaria sp., Pantoea agglomerans, Pseudomonas viridiflava, and Pseudomonas fluorescens were found to be the dominant seed-associated taxa. When assessing microbial function through the perspective of the plant's fitness and survival, several functional characteristics were found to be congruent among seed endophytes. Those include phosphorus solubilization, nitrogen fixation, antibiosis, acetoin secretion, 1- aminocyclopropane-1-carboxylate (ACC) deaminase activity, the production of growth- promoting hormones such as indole acetic acid (IAA), siderophore production to transport iron inside the plant, as well as the ability to suppress growth of plant-pathogenic microorganisms. Interestingly, both members and functions of the seed microbiota are typical for plant-beneficial microbes.
The transmission route of plant endophytes to seeds has been extensively reviewed and can be divided into two groups: sexual or asexual routes. The asexual route includes the vascular system and intracellular cavities connecting the maternal plant to the developing seed through the funiculus, chalaza, and micropyle. The sexual route exclusively includes microorganisms transmitted via male and female gametophytes, which subsequently colonize the embryo and endosperm. The transmission through gametophytes, however, is only hypothesized based on studies showing that flowers, pollen, and ovary contain microbial species similar to those found in seeds. Neither the actual transmission during sexual fertilization nor the subsequent fate and distribution of these microorganisms has been documented.... In the case of oak and melon, the authors showed that embryonic tissues, including the cotyledons, harbored more diverse microbial communities compared to the seed coat.
... A seed comprises the plant embryo, and, depending on the plant species, other compartments such as seed coat, endosperm, and perisperm. These compartments were previously shown to contain distinct communities of microorganisms, some of which can be transmitted to the developing seedling. A recent study on oak showed that seeds transmit a distinct community of microorganisms to the emerging seedling's phyllosphere and roots. This suggests that the seed microbiome contains niche-specialized microbes that then migrate to their respective tissues.
... From a natural perspective, the ecological consequences of inheritance of microorganisms are unexplored, and might turn out to be of major importance for our understanding of spatial and temporal variation in microbiome community structure, plant-microbe interactions, and plant fitness, with consequences for community dynamics and ecosystem functioning. From an evolutionary perspective, microbial inheritance is expected to play a role in evolutionary changes and speciation in both plants and microbes and affect host-symbiont coevolutionary dynamics. From an applied conservation perspective, this offers new ways for the actual topic of ecosystem restoration and microbiome stewardship in preventing biodiversity loss.EXCERPTS from "Introduction": ... To perpetuate their generation, seeds need to survive unfavourable conditions, sense their environment, and synchronize germination with favourable environmental conditions. While seed coat, seed appendages, and endosperm play critical roles in seed biology and ecology, the hitherto often neglected seed microbiome is also an important determinant and driver of seed health and seed germination.... Seeds are not only carriers of the DNA for future generations but also act as a vector to transmit additional functionality in the form of microbes to the next plant generation. The seed microbiome consists of a narrow range of microbial species, which seems to have co-evolved with the host plant species and contributed to plant survival.... Although some information exists about the diversity of the seed microbiome, its functional significance is yet to be fully understood.• 2023 - REVIEW: "Uniting the Role of Endophytic Fungi Against Plant Pathogens and Their Interaction", by Shazia Akram et al., Journal of Fungi.
OTHER EXCERPTS: ... Most of the information about seed-borne microbes and their functional roles has come from culture-based studies on crop plants. Since most of the seed-borne microbes are unculturable, these culture-based methods only document a fraction of the total microbial diversity and hence the seed microbial catalogue remained underexplored and incomplete. This limitation has been overcome due to advancements in molecular techniques and metagenomic Next Generation Sequencing (NGS) which have helped in cataloguing the entire seed microbiome. It has also expanded our knowledge about the diversity of microbes living within seeds and on the seed surface, as well as their putative functional role in the host plants.
... Based on the review of relevant studies undertaken during the present study, the fungal seed diversity is represented by 170 genera belonging to six phyla with the dominance of Ascomycota. The fungi belonged mostly to the classes Dothideomycetes, Eurotiomycetes, Leotiomycetes, Pezizomycetes, and Sordariomycetes.... Fungi of the class Dothideomycetes predominate the fungal seed microbiome and the most abundant genera are Alternaria, Aureobasidium, Cladosporium, Chaetomium, Epicoccum, Fusarium, Microdochium, Phaeosphaeria, and Xylaria. It is noteworthy that arbuscular mycorrhizal fungi do not colonize seeds very often, except for a few Glymeromycota species which also have low prevalence. Compared to the microbiome of other plant parts, only a small fraction of microbial taxa colonizes the seeds.
... Fungal endophytes were traditionally believed to be only horizontally transmitted, but recent studies have shown vertical transmission rates of over 90%, for some fungal endophytes (Hodgson et al., 2014). Moreover, vertical transmission is more effective in asexual fungi. [Editor's note: My understanding is that Fusarium torreyae is asexual when sampled and studied in cankers.] ... Plants have the genetic tendency to accommodate those microbes in their microbiome that promote plant growth and are antagonistic towards soil-borne pathogens.... The microbes present in pollen can be passed on directly through the gametes that finally end up in the embryo and endosperm.
... Seed-associated microbes have been shown to promote the seedling establishment, growth and development of host plants in nutrient-poor soils and under stressful conditions through (1) optimization of nutrient uptake, assimilation through phosphate solubilization, siderophore production, ACC deaminase production and nitrogen fixation; (2) antagonism (e.g., biocontrol of phyto-pathogens) and competition (competitive exclusion of other plant species); (3) improved plant-water-soil relationship; (4) stimulation of plant metabolism (modulation of reactive oxygen species (ROS), auxin, cytokinin and gibberellin levels); and (5) tolerance to abiotic stresses, such as drought, salinity, and stress due to heavy metals. Seed microbiome, therefore, improves the overall fitness of the host and provides it a competitive advantage over other plants, thereby influencing the outcome of competitive interactions in a community and resultantly shaping the community assembly patterns.... In addition, beneficial seed-inhabiting microbes have a profound role in mediating seed dormancy and preparing a suitable environment for germination by managing the hormone levels in the soil.... The underlying mechanisms of germination enhancement are related to the alleviation of stressful conditions by modulating seed hormone levels of gibberellic acid and cytokinins. In addition to seed germination, root growth and drought tolerance under severe drought conditions are attributed to the seed microbiome.... Many plants experience high mortality at the seed stage because they are susceptible to predation by pathogens. The seed microbiome can prevent seed pathogenic microbes from degrading the seeds.... Further, seed-associated microbes may directly produce secondary metabolites or stimulate the production of secondary metabolites by producing elicitors. These secondary metabolites (such as ethylene, jasmonic acid, and salicylic acid) in turn induce host defence against insects or pathogens using induced systemic resistance (ISR) and systemic acquired resistance (SAR) pathways.
Given these growth-promoting roles of seed microbiome, plant species conservation needs a PARADIGM SHIFT from the present approach of plant-specific protocols to envisaging a role for microbiome in the species conservation programmes. It implies that a plant species cannot be conserved without conserving its integral seed microbiome (Berg and Raaijmakers, 2018).... Laboratory experiments do not capture the complexity of seed-microbiome-plant interaction that exists in natural settings.EXCERPTS: Endophytic fungi are used as the most common microbial biological control agents (MBCAs) against phytopathogens and are ubiquitous in all plant parts. Most of the fungal species have roles against a variety of plant pathogens. Fungal endophytes provide different services to be used as pathogen control agents, using an important aspect in the form of enhanced plant growth and induced systemic resistance, produce a variety of antifungal secondary metabolites (lipopeptides, antibiotics and enzymes) through colonization, and compete with other pathogenic microorganisms for growth factors (space and nutrients).BACK TO 6-PART TABLE OF CONTENTS
... The plant can also induce the different genome expression in various endophytic microbes during their colonization. The progress of symbiosis research shows that through nutritional monitoring, plants can identify whether invading microorganisms are beneficial or pathogenic. The genotype of the host plant also significantly affects the endosphere microbiome community in host plants. In some endophytes, in addition to local abiotic stress factors, it has also been found that the change of their lifestyle to a pathogenic state also depends on the genotype of the host (i.e., Ramularia collocygni can survive as an asymptomatic endophyte during the initial growth period, but in the later growth period, it switches to necrotic pathogen). At the ecological time scale, Fusarium verticillioides in maize plant can survive as endophytes or later become a latent pathogen that causes disease over a period of time. However, external and endogenous factors that cause fungi to transform from endophytes to pathogens are not yet fully understood. In order to better understand the dynamics of endophytes, comparative studies are needed to find the gene expression and conditions (in plants and endophytes) in which the same microorganisms behave as symbionts or pathogens. However, for endogenous lifestyles, a single mechanism or factor has not yet been determined, so further discoveries are needed.