Over the last two decades, ash dieback has become a major problem in Europe, where the causative fungus has invaded the continent rapidly. The disease is caused by the invasive pathogenic fungus ...Hymenoscyphus pseudoalbidus (anamorph Chalara fraxinea), which causes severe symptoms and dieback in common ash, Fraxinus excelsior. It is becoming a significant threat to biodiversity in forest ecosystems and the economic and aesthetic impacts are immense. Despite the presence of the disease for at least 10 years in Scandinavia, a small fraction of F. excelsior trees have remained vigorous, and these trees exhibit no or low levels of symptoms even where neighbouring trees are very sick. This gives hope that a fraction of the ash trees will retain a sufficiently viable growth to survive. Following a period of high mortality in natural populations, selection and breeding of remaining viable ash trees could therefore provide a route for restoring the role of ash in the landscape. This paper reviews the available data on disease dissemination, and the consequences thereof in terms of symptom severity and mortality, and appraises studies that have tested the hypothesis that less‐affected trees have genetically based resistance. The implications of the results for the adaptive potential of common ash to respond to the disease through natural or assisted selection are discussed. The risks of adverse fitness effects of population fragmentation due to high mortality are considered. Finally, it is recommended that resistant trees (genotypes) should be selected to facilitate conservation of the species.
Under the rules for the naming of fungi with pleomorphic life-cycles adopted in July 2011, the nomenclaturally correct name for the fungus causing the current ash dieback in Europe is determined to ...be Hymenoscyphus fraxineus, with the basionym Chalara fraxinea, and Hymenoscyphus pseudoalbidus as a taxonomic synonym of H. fraxineus.
This study aimed to demonstrate the association of the ash dieback pathogen Hymenoscyphus fraxineus with leaf symptoms on Fraxinus excelsior and to test its pathogenicity towards leaves of three ...European ash species, F. excelsior, F. angustifolia and F. ornus, in wound inoculation experiments. On F. excelsior, H. fraxineus was isolated from 94% of leaf rachises with necrotic lesions and from 74% of necrotic leaflet midribs. Following wound inoculation of leaf rachises, in two separate experiments performed in 2010 and 2011, the ash dieback pathogen caused symptoms (necrotic rachis lesions, leaf wilting and premature leaf shedding) on all three ash species, while control leaves remained symptomless. Hymenoscyphus fraxineus was consistently reisolated from fungus‐inoculated rachises. All 10 isolates tested were pathogenic to the three ash species and varied in virulence. Koch's postulates for H. fraxineus as causal agent of leaf symptoms on F. excelsior were fulfilled in this study. Complemented with the isolation of the fungus from naturally infected, symptomatic leaf rachises of F. angustifolia and F. ornus in previous investigations, H. fraxineus was confirmed to be a leaf pathogen of these ash species as well. The leaf inoculation experiments showed that F. excelsior was highly susceptible to H. fraxineus, F. angustifolia was equally or slightly less susceptible, whereas F. ornus was the least affected species; however, F. ornus should also be regarded as a host tree for the ash dieback pathogen. This susceptibility ranking corresponds well with field observations and previous stem inoculation experiments.
The future existence of common ash (Fraxinus excelsior), an important tree species throughout temperate Europe, is threatened. An invasive fungal disease (ash dieback) has spread through much of the ...distribution area of common ash. The causal agent of the disease is Hymenoscyphus fraxineus, a necrotrophic ascomycete, most probably introduced from Asia in the early 1990s. Hymenoscyphus fraxineus infects ash trees and saplings through their leaves, from which it grows into the stem. The fungus was studied intensively in recent years but there is still a need to address the topic from an evolutionary perspective. In this overview, some key evolutionary aspects of ash dieback are discussed, from the Red Queen dynamics of host–pathogen interactions to the probable consequences for virulence evolution of multiple infections. The progression of ash dieback in Europe does not show spatial differences, but studies show variation in susceptibility within host populations, a probable consequence of genetic differences, thus providing material for evolution of disease resistance or tolerance. Breeding programmes need to maintain the genetic diversity of Fraxinus, to enable it to withstand further threats such as climate change and the emerald ash borer. Because H. fraxineus reproduces exclusively sexually, the pathogen is likely to overcome a narrow genetic resistance. The introduction of further strains of H. fraxineus to Europe and the movement of infected plant material should be avoided. This case study shows that the integration of evolutionary ecology considerations would benefit plant disease management and biosecurity in general.
Hymenoscyphus fraxineus mitovirus 1 (HfMV1) occurs in the fungus Hymenoscyphus fraxineus, an introduced plant pathogen responsible for the devastating ash dieback epidemic in Europe. Here, we ...explored the prevalence and genetic structure of HfMV1 to elucidate the invasion history of both the virus and the fungal host. A total of 1298 H. fraxineus isolates (181 from Japan and 1117 from Europe) were screened for the presence of this RNA virus and 301 virus‐positive isolates subjected to partial sequence analysis of the viral RNA polymerase gene. Our results indicate a high mean prevalence (78.7%) of HfMV1 across European H. fraxineus isolates, which is supported by the observed high transmission rate (average 83.8%) of the mitovirus into sexual spores of its host. In accordance with an expected founder effect in the introduced population in Europe, only 1.1% of the Japanese isolates were tested virus positive. In Europe, HfMV1 shows low nucleotide diversity but a high number of haplotypes, which seem to be subject to strong purifying selection. Phylogenetic and clustering analysis detected two genetically distinct HfMV1 groups, both present throughout Europe. This pattern supports the hypothesis that only two (mitovirus‐carrying) H. fraxineus individuals were introduced into Europe as previously suggested from the bi‐allelic nature of the fungus. Moreover, our data points to reciprocal mating events between the two introduced individuals, which presumably initiated the ash dieback epidemic in Europe.
A method based on real‐time polymerase chain reaction (PCR) and the use of rotating‐arm spore traps was developed for quantifying airborne Hymenoscyphus pseudoalbidus ascospores. The method was ...sensitive and reproducible, and the collection efficiency was 10% of the spores present in the air. The temporal ascospore dispersal pattern was studied over 3 years by collecting spores every 15 days for a 24 h air‐sampling period during the ash‐growing season. The highest production was detected from the end of June to the beginning of September. The overall ascospore production did not differ significantly among stands within a specific year but there were differences from year to year. There was a positive correlation between air temperature and the number of ascospores trapped, with most of the positive samples being observed at temperatures above 12°C. The vertical profile of ascospore dispersal showed a strong decrease in ascospore density within a height of 3 m, regardless of date of collection. An analysis of the spore traps installed at increasing distances from an infected stand showed that most of the ascospores were deposited downwind within 50 m of the stand. These data are discussed in context of the epidemiology of the disease.
Ash dieback damage was assessed and analysed on 16–22 year-old grafts in two ash seed orchards (Fraxinus excelsior L.). The grafts originated from 106 plus-tree clones selected from 27 stands in ...southern Sweden based on their phenotypes. The results obtained indicate that ash dieback disease is strongly genotypically controlled. There was considerable genotypic variation among individuals. None of the clones seemed to be totally resistant, but some exhibited reduced susceptibility and retained this resistance after 6 years under heavy infection pressure. Autumn phenology based on leaf coloration was subject to moderate genetic control (H² = 0.19). The genetic correlation between autumn phenology and damage was weak to moderate (rG from 0.38 to 0.60) and positive, indicating that susceptible clones have a prolonged growing season. There was no evidence suggesting that stands differed in susceptibility. Together with the high heritability of resistance, strong age×age correlations and weak genotype×environment interactions, this suggests there is good scope for breeding less susceptible trees for the future.
Ash decline induced by Hymenoscyphus pseudoalbidus is an emerging disease that severely affects Fraxinus excelsior stands in Europe. There has been an invasive spread of the disease from east to west ...in Europe over the last decade. Wood discoloration on infected trunks has been reported, but few data are available on the involvement of H. pseudoalbidus in such symptoms. Transport and trade of ash logs could introduce the pathogen into disease‐free areas and therefore accelerate its dissemination. The aim of this study was to assess the prevalence and severity of H. pseudoalbidus in ash logs in infested areas located in the northeast of France and to clarify the role of secondary pathogens in ash decline. The results showed that prevalence of H. pseudoalbidus on collar lesions was high in the study area. The pathogen was able to produce conidia from infected wood. Thus, export of ash logs could represent a potential risk for spreading the disease. Involvement of Armillaria spp. in the decline process was confirmed, while no Phytophthora‐induced collar lesions were found. Studying both disease prevalence and the age of callus tissues surrounding collar lesions in 60 ash stands enabled the origin of the disease in the study area to be determined.
CONTEXT : It might be possible to establish a new generation of Fraxinus excelsior which is insusceptible towards ash dieback (agent: Hymenoscyphus pseudoalbidus) by efficient breeding. However, a ...considerable number of highly tolerant individuals which have the ability to pass on this trait to their progeny are needed. AIMS : The aim of this study was to identify the potential of provenances from southwestern Germany as a source of future selection for resistance or resistance breeding. METHODS : In July 2012 and 2013, ash dieback severity was scored by assessing the crown defoliation and the portion of epicormic shoots in the crowns in clonal seed orchards with a total of 1,726 ash trees in southwestern Germany. RESULTS : Ash dieback severity differed strongly between the orchards and the clones. Broad-sense heritability ranged from 0.18 to 0.55 for crown defoliation and from 0.48 to 0.58 for the portion of epicormic shoots between the orchards. Clones from nearby populations did not show differences in general levels of susceptibility. CONCLUSION : The study highlights that there is high genetic variation in susceptibility and considerable genetic potential for resistance breeding in provenances from southwestern Germany.
Introduced plant pathogens are increasingly recognized as a major threat to biodiversity and ecosystem functioning. One such pathogen, the causal agent of the devastating ash dieback in Europe, ...Hymenoscyphus fraxineus, was most likely introduced into Europe from eastern Asia in the 1990s. To investigate the genetic population structure of this invasive fungus at the epidemic disease front (Switzerland) and in the post-epidemic phase (Lithuania), a total of 847 H. fraxineus isolates were genotyped at 11 microsatellite loci. Among these isolates, 244 multilocus genotypes were found in five post-epidemic subpopulations (367 isolates) of the fungus and 263 in five epidemic subpopulations (480 isolates). No genetic differentiation was found between isolates recovered from bark lesions and fallen leaf petioles, which suggests that all H. fraxineus genotypes have the potential to induce bark infections on living trees and to survive saprophytically. Moreover, no genetic differentiation and no difference in genetic diversity were detected between the epidemic and post-epidemic populations. The entire genetic diversity present in the original founding populations in north-eastern Europe seems to have been transmitted to the epidemic disease front. Nonetheless, gene flow among post-epidemic subpopulations occurs slightly more random than among epidemic subpopulations. Furthermore, the probability of correct assignment of a particular H. fraxineus genotype to its subpopulation of origin was greater in Switzerland than in Lithuania. These two analyses point to weak founder effects at the disease front.
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