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.     

Virulence of Hymenoscyphus albidus and native and introduced Hymenoscyphus fraxineus on Fraxinus excelsior and Fraxinus pennsylvanica

Ash dieback is caused by Hymenoscyphus fraxineus, a cryptic species of the putatively harmless Hymenoscyphus albidus. Recently, H. fraxineus was found to be native to East Asia. However, the virulence of Asian H. fraxineus strains on Fraxinus excelsior and the virulence of European H. albidus on hosts other than F. excelsior and Fraxinus mandshurica have not yet been assessed. In a wound inoculation study, the virulence of four H. albidus and four European and Japanese H. fraxineus strains was assessed on F. excelsior and Fraxinus pennsylvanica in a climate chamber. Lesion lengths were measured after approximately three and a half months. No lesions were observed on the negative control or on trees inoculated with H. albidus. In contrast, inoculation with H. fraxineus induced typical symptoms of ash dieback on both tree species. Japanese H. fraxineus strains induced significantly longer lesions compared to European strains. Fraxinus excelsior was highly susceptible and developed lesions averaging lengths of 1·7 and 8·4 cm for European and Japanese strains, respectively. Fraxinus pennsylvanica was less susceptible and developed average lesion lengths of 1·6 and 4·8 cm for European and Japanese strains, respectively. Most strains were successfully reisolated from necrotic lesions or inocula, fulfilling Koch's postulates. The data show that additional introductions of H. fraxineus strains from the native range to Europe could pose a threat to the conservation of F. excelsior. In addition, introduction of H. fraxineus to North America could potentially have a negative effect on the indigenous F. pennsylvanica.     

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.     

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.     

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.     

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.     

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.     

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.     

A rapid virulence assay for the Dutch elm disease fungus Ophiostoma novo‐ulmi by inoculation of apple (Malus × domestica ‘Golden Delicious’) fruits

Large‐scale virulence tests using trees or saplings are expensive, time‐consuming and require a considerable amount of space. The suitability of using ‘Golden Delicious’ apples as a rapid screen for identifying Ophiostoma novo‐ulmi transformants with reduced virulence was thus evaluated. When a collection of O. novo‐ulmi field isolates belonging to subspecies novo‐ulmi or americana was inoculated to apples, members of subsp. novo‐ulmi induced, on average, larger necrotic lesions than subsp. americana isolates. The size of the lesions on apples was not correlated with mycelial growth rate of isolates on nutrient agar. Insertional mutants from O. novo‐ulmi subsp. novo‐ulmi isolate H327 were inoculated to ‘Golden Delicious’ apples and Ulmus parvifolia × U. americana saplings in parallel experiments. Results clearly indicated that the O. novo‐ulmi transformants included several exhibiting significantly altered levels of virulence. Variability among replicates within a treatment was reduced in apple inoculation data compared to elm sapling data. Overall, the ‘Golden Delicious’ apple assay was found to be an excellent means for rapidly assessing the virulence level of O. novo‐ulmi isolates.     

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.     

An assay for quantitative virulence in Rhynchosporium commune reveals an association between effector genotype and virulence

Detailed knowledge of the evolutionary genetics of virulence is needed to understand and predict host–pathogen dynamics. This study used a virulence assay based on digital image analysis and treated virulence as a quantitative rather than a binary trait. Such quantitative data may better reflect the genetic underpinning of virulence in many pathogen systems and provide better resolution in statistical investigations. A greenhouse experiment based on a common garden design was conducted to measure virulence (% of leaf area covered by lesions) of 126 genetically distinct isolates of the barley scald pathogen, Rhynchosporium commune, originating from nine field populations around the world. Virulence in this pathosystem was found to be a quantitative trait with a continuous distribution in all populations. By comparing population genetic differentiation for virulence and neutral microsatellite markers (i.e. a Q_ST/G_ST comparison), evidence that virulence is under stabilizing selection across populations was found. Heritability values were high and ranged from 0·52 to 0·96 with a mean heritability of 0·84. Virulence was positively correlated with spore production as predicted by the trade‐off theory of virulence evolution. Furthermore, an association analysis between virulence and sequence haplotypes of three known necrosis‐inducing effector genes (NIP1, NIP2 and NIP3) revealed a significant effect of NIP2 haplotypes and NIP1 deletions. Overall, the results support a quantitative model for virulence in the R. commune–barley pathosystem and very high evolutionary potential for this trait.     

Evaluating aggressiveness and host range of Alternaria dauci in a controlled environment

The aggressiveness of Alternaria dauci isolates was investigated in greenhouse conditions. Twenty‐seven isolates were pre‐selected from a large collection to represent high diversity according to geographic or host origins and intergenic spacer (IGS) polymorphism. IGS sequence analysis revealed that isolates were grouped within three different clusters. Eleven isolates were selected and inoculated on a susceptible carrot cultivar. Three criteria (mean lesion number, mean necrotic leaf area and mean disease index) were used to assess the aggressiveness of isolates. Continuous variation in aggressiveness was shown and no clear division into isolate classes was evident. For the host range study, two isolates were inoculated under greenhouse conditions onto nine cultivated Apiaceae species, two wild Daucus species and six cultivated non‐Apiaceae species representing six botanical families. Lesions varying in severity were observed on all dicot species (Apiaceae and non‐Apiaceae), but no symptoms developed on the two monocots studied (leek and sweetcorn). Plant species were also differentiated on the basis of expanding lesions (cultivated and wild carrot, dill and fennel) or non‐expanding lesions (other dicot species). Typical A. dauci conidia were observed after in vitro incubation of leaves with symptoms. Fungal structures were isolated from lesions and A. dauci was confirmed on the basis of conidial morphology and specific conventional PCR results. Genotyping of individual isolates performed with microsatellite markers confirmed the presence of the inoculated isolate. The results clearly showed that, in controlled conditions, the host range of A. dauci is not restricted to carrot.     

Phytophthora acerina sp. nov., a new species causing bleeding cankers and dieback of Acer pseudoplatanus trees in planted forests in northern Italy

A severe dieback of Acer pseudoplatanus trees was noticed in planted forest stands in northern Italy in 2010. Affected trees showed collar rot and aerial bleeding cankers along the stems, leading to crown dieback and eventually death. An unknown Phytophthora species was consistently isolated from necrotic bark and xylem tissue and from rhizosphere soil. Based on its unique combination of morphological and physiological characters and phylogenetic analysis, this new taxon is here described as Phytophthora acerina sp. nov. Phylogenetic analysis of ITS, cox1 and β‐tubulin gene regions demonstrated that P. acerina is unique and forms a separate cluster within the ‘P. citricola complex’, closely related to P. plurivora. Phytophthora acerina is homothallic with smooth‐walled oogonia, thick‐walled, mostly aplerotic oospores with a high abortion rate, paragynous antheridia, and persistent, morphologically variable semipapillate sporangia. Four to 5‐week‐old cultures produced globose to subglobose, appressoria‐like and coralloid hyphal swellings and characteristic stromata‐like hyphal aggregations. Optimum and maximum temperatures for growth are 25°C and 32°C, respectively. Genetic uniformity of all 15 studied isolates and the apparent absence of this species in the extensive surveys of nurseries, forests and seminatural ecosystems conducted in the previous two decades across Europe indicate a recent clonal introduction to northern Italy. Under‐bark inoculation tests demonstrated high aggressiveness of P. acerina to A. pseudoplatanus indicating that this pathogen might be a serious risk to maple plantations and forests in Europe.     

Genetics of virulence in Ascochyta rabiei

In order to critically test the hypothesis that virulence variation in the Ascochyta rabiei/chickpea pathosystem is a discrete character under simple genetic control, a genetic cross was made between a highly virulent isolate of A. rabiei from Syria and a less virulent isolate from the USA. Two independent virulence assays conducted by inoculating susceptible and resistant chickpea cultivars under controlled conditions with 77 independent progeny isolates from this cross revealed a continuous distribution of disease phenotypes. Bimodality, as would be predicted for the segregation of virulence under simple genetic control, was not supported by statistical tests of the progeny phenotype distribution. anova revealed highly significant pathogen‐genotype × host‐genotype interactions demonstrating the segregation of genes controlling specialization on the two cultivars tested. These interactions could be localized to two isolates that changed virulence rank on the cultivars. It was concluded that variation in virulence to these two cultivars is under quantitative genetic control. If this conclusion applies to other cultivars, it can be speculated that the discrete categories of virulence variation identified in previous studies were probably the result of incomplete sampling of host resistance or pathogen virulence variation and/or of selection for increased virulence in contemporary A. rabiei populations.     

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.     

Diaporthe sclerotioides exhibits no host specificity among cucurbit species

Plant–fungal specificity between cucurbitaceous crops and Diaporthe sclerotioides, the causal agent of black root rot, was studied using cucumbers (Cucumis sativa), melons (Cucumis melo), pumpkins (Cucurbita maxima), watermelons (Citrullus lanatus) and bottlegourd (Lagenaria siceraria var. gourda). Twelve D. sclerotioides isolates from these cucurbit species were cross‐inoculated. The virulence of the isolates was evaluated as the area under the disease progress curve (AUDPC). All cucurbit species were susceptible to each isolate, but AUDPCs were significantly different among the hosts, with the order of greatest to least being melon, cucumber, watermelon, bottlegourd and finally, pumpkin. The infectiveness of isolates was assessed as the quantity of D. sclerotioides DNA detected in the hypocotyls of seedlings 2 weeks after inoculation using a real‐time PCR protocol. The fungal DNA quantities varied among the species in the same order as the AUDPCs. Whilst there were statistically significant correlations between the virulence and infectiveness of D. sclerotioides isolates in cucumbers, melons and bottlegourds, their coefficients of determination were not high (r² < 0·6). Orthogonal contrasts indicated no specificity in either the fungal virulence or infectiveness between D. sclerotioides isolates and the cucurbit hosts from which these isolates originated. Thus, although the degree of host susceptibility to D. sclerotioides varies among cucurbit species, the absence of specificity to the host species in either virulence or infectiveness suggests the pathogen may spread via various cucurbit crops, irrespective of their original host species.     

Russian populations of Puccinia triticina in distant regions are not differentiated for virulence and molecular genotype

The objective of this study was to determine whether genetically distinct groups of Puccinia triticina are present in four regions of the Russian Federation. Collections of P. triticina were obtained from the Central, North Caucasus, Volga and West Siberia regions from 2006 to 2010. Ninety‐nine single uredinial isolates were tested for virulence phenotype with 20 Thatcher near‐isogenic lines of wheat. Forty‐one virulence phenotypes were found in the four regions, with eight in common between the widely separated Central and West Siberia regions. A total of 72 isolates were tested for molecular genotype with 23 simple sequence repeat (SSR) primer pairs, and 66 isolates were used for further analysis after clone correction for virulence and molecular genotype. Analysis of variation showed no overall differentiation of SSR genotypes or virulence phenotypes based on region of origin. Linkage disequilibria for SSR genotypes were high across the entire population. The regional populations had higher than expected levels of allelic heterozygosity that indicated clonal reproduction. Based on cluster analysis of SSR genotypes there were two groups of P. triticina isolates that were widely distributed across Russia. The two SSR groups also differed significantly for virulence. Puccinia triticina may be dispersed from a common source of inoculum in the European or Caucasus regions of Russia. The Russian P. triticina populations were highly differentiated for SSR genotype from populations in Tajikistan, Kyrgyzstan, Uzbekistan, Armenia, Georgia and Azerbaijan and more similar to populations from southern Kazakhstan and northern Kazakhstan.     

Morphological and molecular diversity of Colletotrichum spp. causing pepper spot and anthracnose of lychee (Litchi chinensis) in Australia

Since the 1980s a new disease has been affecting Australian lychee. Pepper spot appears as small, black superficial lesions on fruit, leaves, petioles and pedicels and is caused by Colletotrichum gloeosporioides, the same fungus that causes postharvest anthracnose of lychee fruit. The aim of this study was to determine if a new genotype of C. gloeosporioides is responsible for the pepper spot symptom. Morphological assessments, arbitrarily‐primed PCR (ap‐PCR) and DNA sequencing studies did not differentiate isolates of C. gloeosporioides from anthracnose and pepper spot lesions. The ap‐PCR identified 21 different genotypes of C. gloeosporioides, three of which were predominant. A specific genotype identified using ap‐PCR was associated with the production of the teleomorph in culture. Analysis of sequence data of ITS and β‐tubulin regions of representative isolates did not group the lychee isolates into a monophyletic clade; however, given the majority of the isolates were from one of three genotypes found using ap‐PCR, the possibility of a lychee specific group of C. gloeosporioides is discussed.     

Genetic and phenotypic characterization of Botrytis calthae

Botrytis calthae is a necrotrophic plant pathogen, closely related to the ubiquitous broad host range fungus Botrytis cinerea, but highly host specific. Botrytis isolates from lesions of Caltha palustris grown at different locations were classified with genetic markers as either B. calthae or Botrytis pseudocinerea, or less frequently as B. cinerea. A PCR‐based identification of B. calthae was developed. Seven haplotypes of B. calthae could be distinguished. Compared to B. cinerea, mycelium growth of B. calthae was similar, but conidiation less abundant, and sclerotia formation was only partially repressed by light. Conidia of B. calthae germinated more slowly, and showed a highly acidic optimum (pH 2·5) compared to B. cinerea conidia (pH 5·3). All B. calthae isolates were sensitive to common anti‐Botrytis fungicides, but showed partial resistance to the succinate dehydrogenase inhibitors boscalid, fluopyram and carboxin. Infection experiments revealed a weak capability of B. calthae to induce necrotic lesions on plants that are hosts for B. cinerea. On C. palustris leaves, B. calthae induced similar lesions to B. cinerea. These data provide a basis for comparative molecular investigation of the physiology and host specificity of B. calthae and closely related Botrytis species.