Pathogenicity of Hymenoscyphus fraxineus towards leaves of three European ash species: Fraxinus excelsior,F. angustifolia and F. ornus

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.     

Hymenoscyphus fraxineus is a leaf pathogen of local Fraxinus species in the Russian Far East

Dieback of European ash was first observed in Europe in the early 1990s. The disease is caused by the invasive ascomycete Hymenoscyphus fraxineus, proposed to originate from Far East Asia, where it has been considered a harmless saprotroph. This study investigates the occurrence of H. fraxineus in tissues of local ash species in the Russian Far East, and assesses its population‐specific genetic variation by ITS sequencing. Shoot dieback symptoms, characteristic of H. fraxineus infection on European ash, were common, but not abundant, on Fraxinus mandshurica and Fraxinus rhynchophylla trees in Far East Russia. High levels of pathogen DNA were associated with necrotic leaf tissues of these ash species, indicating that the local H. fraxineus population is pathogenic to their leaves. However, the low levels of H. fraxineus DNA detected in shoots with symptoms, the failure to isolate this fungus from such tissues, and the presence of other fungi with pathogenic potential in shoots with symptoms indicate that local H. fraxineus strains may not be responsible (or their role is negligible) for the observed ash shoot dieback symptoms in the region. Conspicuous differences in ITS rDNA sequences detected between H. fraxineus isolates from Russian Far East and European populations suggest that the current ash dieback epidemic in Europe might not directly originate from the Russian Far East. Revision of the herbarium material shows that the earliest specimen of H. fraxineus was collected in 1962 from the Russian Far East and the oldest H. fraxineus specimen of China was collected in 2004.     

Pathogenicity of fungi associated with ash dieback towards Fraxinus excelsior

A large part of the area in Europe in which Fraxinus excelsior is native is currently affected by ash dieback, a threatening disease caused by the ascomycetous fungus Hymenoscyphus fraxineus. Fungi other than H. fraxineus also occur in large numbers on stems of the dying ash trees. To clarify their possible role in the dieback process, six fungal species common on dying stems and twigs of ash in Poland, i.e. Cytospora pruinosa, Diaporthe eres, Diplodia mutila, Fusarium avenaceum, F. lateritium and F. solani, were tested for pathogenicity using a test based on artificial wound inoculations of 6‐year‐old F. excelsior plants under field conditions, with H. fraxineus included for comparison. There were significant differences in index of pathogenicity among the fungi tested. Hymenoscyphus fraxineus (mean index 5.78) was the most pathogenic. Diplodia mutila (4.23) and C. pruinosa (4.02) were significantly less pathogenic than H. fraxineus, but significantly more than the other fungi. Diaporthe eres (2.43), F. avenaceum (1.92), F. solani (1.86) and F. lateritium (1.08) were the least pathogenic (P < 0.0001). The extent of disease symptoms caused by F. solani and F. lateritium was statistically similar to the control (P = 0.05). All tested fungi were successfully reisolated from inoculated stems. The contribution of the results to understanding the possible role of these fungi in the ash dieback process in F. excelsior, particularly in trees weakened after primary infection by H. fraxineus, is discussed.     

Virulence of the invasive ash pathogen Hymenoscyphus fraxineus in old and recently established populations

Following its introduction from Asia in the 1990s, the ascomycete Hymenoscyphus fraxineus has caused a severe dieback of Fraxinus excelsior in Europe. In this study, the virulence of 200 H. fraxineus isolates were assessed and compared. These isolates equally represented (i) two geographically distant populations with a different disease history (Switzerland, recently established populations at the epidemic front versus Lithuania, old established populations), and (ii) isolates from two different types of host tissue (necrotic bark lesions as dead‐end tissue versus fallen leaf petioles as primary host tissue). Inoculations conducted on 3‐year‐old F. excelsior seedlings showed that the vast majority of the isolates (98%) were able to induce necrotic bark lesions after 10 months. Although a high variation in virulence was observed among isolates, no significant differences were detected between the older and the epidemic‐front populations. Decline in virulence of populations of invasive organisms is generally assumed with increasing age of epidemics. However, this does not appear to hold true for H. fraxineus. Either the Lithuanian population is still too young (15 to 20 years old) to show a decline in virulence, or the size of the host population may still not be critical for pathogen survival. Given that bark lesions represent an epidemiological dead end and do not benefit the survival of H. fraxineus, a trend towards reduced ‘bark virulence’ of isolates originating from leaf petioles compared to isolates from the bark lesions was expected. However, such a trend was observed neither in old, nor in recently established populations.     

Ash dieback due to Hymenoscyphus fraxineus: what can be learnt from evolutionary ecology?

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 species associated with European ash1

The causative agent of dieback on European ash (Fraxinus excelsior) was first described as Chalara fraxinea based on cultural morphology because no sexual stage of the fungus was known. Later, based on culturing of ascospores of a candidate teleomorph, morphological comparison and nuclear ribosomal internal transcribed spacer sequencing, the sexual stage of C. fraxinea was assigned as Hymenoscyphus albidus, a native and widespread species in Europe. Recently, the morphological species concept of H. albidus was shown to cover two species that cannot be separated from each other based on teleomorph characters, but which can be distinguished by several DNA markers. As a result, the strains causing ash dieback were reassigned as H. pseudoalbidus. The closely related H. albidus is presumably a non‐pathogenic endophyte, but pathogenicity tests to confirm this hypothesis have not yet been performed. Genotyping of herbarium specimens has shown that H. pseudoalbidus was present in Switzerland for at least a decade prior to the epidemic outbreak in Europe. The origin of the ash dieback pathogen, and the general importance of correct pathogen identification to development of effective disease control, are discussed.     

Variation in aggressiveness of Hymenoscyphus fraxineus genotypes amid the ash dieback epidemic

The ascomycete Hymenoscyphus fraxineus causes devastating damage to the European common ash (Fraxinus excelsior). The fungus originates from Asia, where it coexists with native ash species and completes its life cycle by sporulating on degrading ash leaf litter on the forest floor. Given this life cycle of the fungus, genotypes of H. fraxineus with varying degrees of aggressiveness may coexist in infected European ash forests. To test this hypothesis, we cultured 19 single-spore isolates from apothecia collected in a trial of heavily infected ash in Denmark and carried out stem inoculations on young ash seedlings. Microsatellite markers revealed that the 19 isolates were all genetically unique and did not show any genetic structure. High variation was observed among the 19 isolates in pathogenicity on young seedlings. The variation (assessed as necrosis development) was highly significant, but not correlated with the vigour of the fungal isolates when grown on culture media. The results support the hypothesis that aggressiveness of H. fraxineus may not be crucial for its fitness. In this sense, ash dieback disease may differ from other recent emerging infectious diseases on trees. We discuss the variation in aggressiveness of H. fraxineus in relation to durability of resistance and future management of the ash dieback epidemic. The findings of this study call for more research into natural variation in endophytic versus pathogenic behaviour of H. fraxineus on European ash.     

Light and scanning electron microscopy studies of the early infection stages of Hymenoscyphus pseudoalbidus on Fraxinus excelsior

The ontogeny and morphology of infection structures associated with the early stages of infection of Hymenoscyphus pseudoalbidus on common ash (Fraxinus excelsior) leaves and leaf petioles were investigated using light and scanning electron microscopy. Ascospores were produced in mature ascocarps and infections on ash leaves were first observed 2 weeks later. Ascospores developed germ tubes, followed by appressorium formation and penetration of epidermal cells on ash leaves and petioles. Chalara fraxinea spores, the anamorph of H. pseudoalbidus, appeared and were arranged in chains, surrounded by a membranous sheath, and varied considerably in size and shape. Host invasion and colonization of all cell types of leaves and petioles were observed using light microscopy. The role of leaves and petioles as sites of infection in the life cycle of H. pseudoalbidus and the disease cycle of ash dieback is discussed.     

Population structure of the ash dieback pathogen,Hymenoscyphus fraxineus,in relation to its mode of arrival in the UK

The ash dieback fungus, Hymenoscyphus fraxineus, a destructive, alien pathogen of common ash (Fraxinus excelsior), has spread across Europe over the past 25 years and was first observed in the UK in 2012. To investigate the relationship of the pathogen's population structure to its mode of arrival, isolates were obtained from locations in England and Wales, either where established natural populations of ash had been infected by wind‐dispersed ascospores or where the fungus had been introduced on imported planting stock. Population structure was determined by tests for vegetative compatibility (VC), mating type and single‐nucleotide polymorphisms (SNPs). VC heterogeneity was high at all locations, with 96% of isolate pairings being incompatible. Frequencies of the MAT1‐1‐1 and MAT1‐2‐1 idiomorphs were approximately equal, consistent with H. fraxineus being an obligate outbreeder. Most SNP variation occurred within study location and there was little genetic differentiation between the two types of location in the UK, or between pathogen populations in the UK and continental Europe. There was modest differentiation between UK subpopulations, consistent with genetic variation between source populations in continental Europe. However, there was no evidence of strong founder effects, indicating that numerous individuals of H. fraxineus initiated infection at each location, regardless of the route of pathogen transmission. The ssRNA virus HfMV1 was present at moderate to high frequencies in all UK subpopulations. The results imply that management of an introduced plant pathogen requires action against its spread at the continental level involving coordinated efforts by European countries.     

Influence of site and stand factors on Hymenoscyphus fraxineus‐induced basal lesions

Hymenoscyphus fraxineus is an invasive fungus in Europe and causes a severe decline affecting ash, which began in the late 1990s. One of the symptoms associated with the disease is lesions in the outer bark of the collar area. However, the aetiology of these basal lesions, and in particular the relative roles of H. fraxineus and Armillaria species, is still controversial; moreover, little is known about the influence of environmental factors on the disease epidemiology. This study therefore surveyed 42 plots located in northeastern France, in an area affected by ash decline since 2008, in order to determine which environmental factors condition the severity of lesions associated with H. fraxineus on ash collar. The spatial pattern that is a consequence of the invasive spread of the disease was taken into account in the analysis, using a spatial hierarchical Bayesian model fitted by integrated nested laplace approximation (INLA). Results show that while basal lesions are tightly associated with H. fraxineus, their severity is influenced by the Armillaria species present in the plot. Sites with vegetation indicating moist conditions, or more humid topographical positions, were associated with more developed basal lesions.     

The ash dieback crisis: genetic variation in resistance can prove a long‐term solution

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.     

Occurrence of Hymenoscyphus pseudoalbidus on infected ash logs

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.     

Evaluation and validation of a loop-mediated isothermal amplification test kit for detection of <Emphasis Type="Italic">Hymenoscyphus fraxineus</Emphasis>

Hymenoscyphus fraxineus (anamorph Chalara fraxinea) is the ascomycete fungus which causes ash dieback, a potentially lethal disease of Fraxinus excelsior in Europe. Isolation and culturing of H. fraxineus is time consuming and there is a need for rapid, specific diagnostic tools to assist in the deployment of appropriate phytosanitary measures. Loop-mediated isothermal amplification (LAMP) is a nucleic acid amplification method which can be used for on-site diagnosis. OptiGene have recently released a kit for rapid in planta detection of H. fraxineus using LAMP. The performance of the kit was evaluated for use in the laboratory and in the field in a comparison with real-time PCR, and the kit was also validated according to EPPO standard 7/98. The analytical sensitivity of the kit was found to be 7 pg of DNA extracted from pure culture. The kit was rapid, with an average time to positive of 15.5 min for field samples. A comparison of real-time PCR and LAMP was carried out in the laboratory and in the field. The diagnostic sensitivity and specificity of the LAMP kit in the laboratory were 90.2% and 98.0% respectively, and 98.3% and 88.6% in the field. Positive and negative predictive values and likelihood ratios were also calculated and considered in relation to the prevalence of the disease. The kit was found to be a useful diagnostic tool which can be applied in the field.     

Ash dieback: pathogen spread and diurnal patterns of ascospore dispersal,with special emphasis on Norway*

Dieback of European ash (Fraxinus excelsior), caused by the ascomycete Hymenoscyphus pseudoalbidus (anamorph Chalara fraxinea), started around 1992 in Poland and has since then spread over large geographical areas. By November 2010, the disease had been recorded in 22 European countries. The gradual expansion and high intensity of the ash dieback epidemic in Europe may suggest that H. pseudoalbidus is an invasive alien organism. In Norway, ash dieback was first reported in spring 2008, and a survey in early summer of the same year revealed that the disease had spread over large parts of the southern and eastern regions of the country. The distance from the southernmost to the northernmost infected stands was, at that time, about 400 km. Some old necrotic lesions were also observed, indicating that the ash dieback pathogen is likely to have been present in Norway since at least 2006. In 2009, a spore sampler was installed in a diseased ash stand at Ås, South‐Eastern Norway. Sampling started in late July and continued until late September. Large numbers of ascospores resembling those of H. pseudoalbidus were observed, with the maximum number of spores occurring from the end of July to mid‐August. The deposition of ascospores occurred mainly between 6 and 8 a.m. Ascospores are most likely to be the primary source initiating host infections and responsible for the rapid recent spread of H. pseudoalbidus in Europe.     

The use of ascospores of the dieback fungus Hymenoscyphus fraxineus for infection assays reveals a significant period of biotrophic interaction in penetrated ash cells

Ascospores, discharged naturally from apothecia growing on rachis debris, were used as inoculum to examine the invasion of ash tissues by Hymenoscyphus fraxineus in order to understand the critical, but poorly understood, early interactions between host and pathogen. Methods were developed to collect ascospores for controlled infection assays on detached leaves, petioles and stem internode tissues. Light microscopy, using plasmolytic techniques, allowed the invasion of living plant cells to be observed. Ascospores were readily available from late May to September. On the plant surface, most spores differentiated directly to form appressoria without germ‐tube growth. Direct penetration was followed by a significant period of biotrophic fungal growth before lesions developed. Following the formation of a vesicle‐like structure after penetration, bulbous and elongated intracellular hyphae were produced in living plant cells. The use of ascospore inoculum, rather than mycelia, will allow natural and rapid screening of ash genotypes for resistance to the devastating dieback disease. The identification of the biotrophic phase of infection suggests that host range is controlled by effector‐triggered immunity.     

Inheritance of virulence in Fusarium circinatum,the cause of pitch canker in pines

Wildtype strains of Fusarium circinatum, the causal agent of pitch canker, were crossed to obtain an F₁ generation. Progeny of this cross were tested for virulence by inoculating Pinus radiata seedlings, and were found to induce a wide range of lesion lengths. Two strains from the F₁ generation that induced long lesions (= high virulence) were used as parents to produce an F₂ generation, followed by a second round of selection for high virulence to obtain an F₃ generation. Mean lesion lengths were not significantly different between the three generations (P ≥ 0.196). A parallel set of crosses was performed to select for low virulence by using progeny in the F₁ and F₂ generations that induced short lesions as parents for F₂ and F₃ generations, respectively. In this case, both rounds of selection resulted in a significant reduction in mean lesion length, from 33.8 ± 0.8 mm in the F₁ generation, to 19.7 ± 0.7 and 12.9 ± 0.7 mm in the F₂ and F₃ generations, respectively. Thus it is apparent that F. circinatum retains the genetic capacity for avirulence to pines, which could reflect a lack of strong selection for virulence in nature. Progeny of a cross between high and low virulence parents manifested nearly continuous variation in lesion lengths, consistent with virulence being a quantitatively inherited trait. Based on this cross, broad‐sense heritability (H²) was determined to be 0.74, which suggests that virulence is under strong genetic control.     

Assessing host specialization of Botrytis cinerea on lettuce and tomato by genotypic and phenotypic characterization

This study tested the hypothesis that Botyrtis cinerea shows host specialization on tomato and lettuce, using phenotypic and genotypic tools. Strains were isolated from tomato and lettuce grown together in the same greenhouse. Forty‐four lettuce strains and 42 tomato strains were investigated for their genetic diversity and their aggressiveness. Both gene diversity and allelic richness were significantly higher in lettuce strains than in tomato strains (P = 0·01). Cluster analysis revealed a clear division of the strains under study into two clusters. However, this structure did not separate the strains according to their host of origin. Tomato strains were significantly more aggressive than lettuce strains when inoculated on tomatoes (P = 0·001), but no significant differences in aggressiveness were observed when the strains were inoculated on lettuce (P = 0·17) or on apple (P = 0·87). The results suggest an absence of clear host specialization of B. cinerea on tomato and lettuce.     

Species composition,toxigenic potential and pathogenicity of Fusarium graminearum species complex isolates from southern Brazilian rice

This study aimed to assess the extent and distribution of Fusarium graminearum species complex (FGSC) diversity in rice seeds produced in southern Brazil. Four species and two trichothecene genotypes were detected among 89 FGSC isolates, based on a multilocus genotyping assay: F. asiaticum (69·6%) with the nivalenol (NIV) genotype, F. graminearum (14·6%) with the 15‐acetyldeoxynivalenol (ADON) genotype, and F. cortaderiae (14·6%) and F. meridionale (1·1%), both with the NIV genotype. Seven selected F. asiaticum isolates from rice produced NIV in rice‐based substrate in vitro, at levels ranging from 4·7 to 84·1 μg g^?1. Similarly, two F. graminearum isolates from rice produced mainly 15‐ADON (c. 15–41 μg g^?1) and a smaller amount of 3‐ADON (c. 6–12 μg g^?1). One F. meridionale and two F. cortaderiae isolates did not produce detectable levels of trichothecenes. Two F. asiaticum isolates from rice and two from wheat (from a previous study), and one F. graminearum isolate from wheat, were pathogenic to both crops at various levels of aggressiveness based on measures of disease severity in wheat spikes and rice kernel infection in a greenhouse assay. Fusarium asiaticum and the reference F. graminearum isolate from wheat produced NIV, and deoxynivalenol and acetylates, respectively, in the kernels of inoculated wheat heads. No trichothecene was produced in kernels from inoculated rice panicles by any of the isolates. These findings constitute the first report of FGSC composition in rice outside Asia, and confirm the dominance of F. asiaticum in rice agroecosystems.     

Biocontrol and population dynamics of Fusarium spp. on wheat stubble in Argentina

The biocontrol effect of Clonostachys rosea (strains 016 and 1457) on Fusarium graminearum, F. avenaceum, F. verticillioides, F. langsethiae, F. poae, F. sporotrichioides, F. culmorum and Microdochium nivale was evaluated on naturally infected wheat stalks exposed to field conditions for 180 days. Experiments were conducted at two locations in Argentina, Marcos Juarez and Río Cuarto. Antagonists were applied as conidial suspensions at two inoculum levels. Pathogens were quantified by TaqMan real‐time qPCR. During the first year at Marcos Juarez, biocontrol was observed in one antagonist treatment for F. graminearum after 90 days (73% reduction) but after 180 days, the pathogen decreased to undetectable levels. During the second year, biocontrol was observed in three antagonist treatments for F. graminearum and F. avenaceum (68·3% and 98·9% DNA reduction, respectively, after 90 days). Fusarium verticillioides was not controlled at Marcos Juarez. At Río Cuarto, biocontrol effects were observed in several treatments at different intervals, with a mean DNA reduction of 88·7% for F. graminearum and F. avenaceum, and 100% reduction for F. verticillioides in two treatments after 180 days. Populations of F. avenaceum and F. verticillioides were stable; meanwhile, F. graminearum population levels varied during the first 90 days, and low levels were observed after 180 days. The other pathogens were not detected. The study showed that wheat stalks were important reservoirs for F. avenaceum and F. verticillioides populations but less favourable for F. graminearum survival. Clonostachys rosea (strain 1457) showed potential to reduce the Fusarium spp. on wheat stalks.     

Detection and quantification of airborne inoculum of Hymenoscyphus pseudoalbidus using real‐time PCR assays

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.