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.     

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.     

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.     

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.     

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.     

Phenotypic,molecular and pathogenic characterization of Phlyctema vagabunda,causal agent of olive leprosy

Olive leprosy, caused by the fungus Phlyctema vagabunda, is a classic fruit rot disease widespread in the Mediterranean basin. From 2009 to 2013, new disease symptoms consisting of small circular necrotic leaf lesions, coin branch canker and shoot dieback were observed in Spanish and Portuguese olive orchards showing intense defoliation. Phlyctema‐like anamorphs were consistently isolated from leaves and shoots with symptoms. Representative isolates from affected leaves, shoots and fruits were characterized based on morphology of colonies and conidia, optimum growth temperature and comparison of DNA sequence data from four regions: ITS, tub2, MIT and rpb2. In addition, pathogenicity tests were performed on apple and olive fruits, and on branches and leaves of olive trees. Maximum mycelial growth rate ranged between 0.54 and 0.73 mm per day. Conidia produced on inoculated apple fruits showed slight differences in morphology among the representative fungal isolates evaluated. Phylogenetic analysis clustered all of the Phlyctema‐like isolates in the same clade, identifying them as Phlyctema vagabunda. On fruits, influence of wounding, ripening and cultivar resistance was studied, with cv. Blanqueta being the most susceptible cultivar. On branches, a mycelial‐plug inoculation method reproduced olive leprosy symptoms and caused shoot dieback. On leaves, Koch's postulates were fulfilled and the pathogen caused characteristic necrotic spots and plant defoliation. This is the first time that the pathogenicity of P. vagabunda in olive leaves has been demonstrated.     

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.     

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.     

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: 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.     

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.     

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.     

Lack of host specificity of Colletotrichum spp. isolates associated with anthracnose symptoms on mango in Brazil

The aim of the present study was to analyse the genetic and pathogenic variability of Colletotrichum spp. isolates from various organs and cultivars of mango with anthracnose symptoms, collected from different municipalities of São Paulo State, Brazil. Colletotrichum gloeosporioides isolates from symptomless citrus leaves and C. acutatum isolates from citrus flowers with post‐bloom fruit drop symptoms were included as controls. Sequencing of the ITS region allowed the identification of 183 C. gloeosporioides isolates from mango; only one isolate was identified as C. acutatum. amova analysis of ITS sequences showed larger genetic variability among isolates from the same municipality than among those from different populations. fAFLP markers indicated high levels of genetic variability among the C. gloeosporioides isolates from mango and no correlation between genetic variability and isolate source. Only one C. gloeosporioides mango isolate had the same genotype as the C. gloeosporioides isolates from citrus leaves, as determined by ITS sequencing and fAFLP analysis. Pathogenicity tests revealed that C. gloeosporioides and C. acutatum isolates from either mango or citrus can cause anthracnose symptoms on leaves of mango cvs Palmer and Tommy Atkins and blossom blight symptoms in citrus flowers. These outcomes indicate a lack of host specificity of the Colletotrichum species and suggest the possibility of host migration.     

Characterization of Cytospora isolates from wood cankers of declining grapevine in North America,with the descriptions of two new Cytospora species

Cytospora species are ubiquitous pathogens of numerous woody plants, causing dieback and wood cankers in agronomic crops, timber trees and wildland trees (e.g. Prunus, Eucalyptus and Salix, respectively). Cytospora chrysosperma, C. cincta and C. leucostoma have been reported from grapevines in Iran showing symptoms of one or more recognized trunk diseases (esca, botryosphaeria‐, eutypa‐ and phomopsis diebacks); however, only C. chrysosperma was shown to be pathogenic to grapevine. To understand the potential role of Cytospora species in the grapevine trunk‐disease complex, 21 Cytospora isolates were examined that were recovered from dieback and wood cankers of Vitis vinifera and Vitis interspecific hybrids in seven northeastern U.S. states and two Canadian provinces. Phylogenetic analyses of ITS and translation elongation factor 1‐α identified two novel species: Cytospora vinacea sp. nov. and Cytospora viticola sp. nov. Differences in culture morphology and conidial dimensions also distinguished the species. When inoculated to the woody stems of potted V. vinifera ‘Thompson Seedless’ in the greenhouse, both species were pathogenic, based on development of wood lesions and fulfilment of Koch's postulates. Cytospora viticola was the most virulent based on lesion length at 12 months post‐inoculation. As cytospora canker shares some of the same general dieback‐type symptoms as botryosphaeria‐, eutypa‐ and phomopsis diebacks, it may be considered part of the grapevine trunk‐disease complex in eastern North America.     

Phytophthora oleae sp. nov. causing fruit rot of olive in southern Italy

A homothallic Phytophthora species was found to be consistently associated with a rot of mature fruits of two local cultivars of olive (Olea europaea) in Calabria, southern Italy. The phylogenetic analysis of sequences of the ITS1‐5.8S‐ITS2 region and cox1 gene enabled its identification as a new species of clade 2, with a basal position compared to previously described subclades. The new species is described formally with the epithet Phytophthora oleae, referring to the natural matrix from which it was isolated. A unique combination of molecular and morphological characters clearly separates P. oleae from other already described Phytophthora species. This new species produced semipapillate, occasionally bipapillate, persistent sporangia on simple sympodially branching sporangiophores as well as globose and smooth‐walled oogonia, paragynous antheridia and spherical, plerotic oospores. The pathogenicity of P. oleae was confirmed in inoculation trials on fruits of three olive cultivars, including the two local cultivars from which the pathogen had been isolated.     

Occurrence and ITS diversity of wood‐associated Bursaphelenchus nematodes in Scots pine forests in Switzerland

The quarantine pathogen Bursaphelenchus xylophilus (pine wood nematode, PWN) represents a serious threat for Pinus species in Europe. To exclude its presence in Switzerland, in 2010 and 2011 a countrywide survey was conducted in 102 Pinus sylvestris stands, chosen according to whether they contained dying or dead trees or were located in areas at risk of PWN introduction. In total, 285 trees (1–5 per site) were sampled. Nematodes were extracted from wood chips using a standard procedure, and identified to species by internal transcribed spacer (ITS) sequencing. Bursaphelenchus species were present in 34% of the trees, but no B. xylophilus was identified, i.e. PWN is still not present in Switzerland. The nematodes found belonged to seven different species, with B. vallesianus the most frequent species, followed by B. sexdentati, B. mucronatus kolymensis and B. eggersi. Three other species (B. borealis, B. pinophilus, B. poligraphi) were each only present in one or two trees. Three groups of sequences could not be assigned to a species because of the lack of matching reference sequences. The species composition found in Switzerland suggests co‐existence of southern and central European Bursaphelenchus species. Intraspecific ITS variability differed considerably among the four most common species. Bursaphelenchus eggersi, B. mucronatus kolymensis and B. sexdentati had several variable sites in the ITS region, resulting in multiple ITS genotypes in each species. In contrast, all 99 B. vallesianus isolates had an identical ITS region. This could indicate a founder effect, and possibly that B. vallesianus is not native to the Alpine region.     

Development of three fusarium crown rot causal agents and systemic translocation of deoxynivalenol following stem base infection of soft wheat

Fusarium pseudograminearum, F. culmorum and F. graminearum are the most important fusarium crown rot (FCR) causal agents. They have the common ability to biosynthesize deoxynivalenol (DON). To elucidate the behaviour of each of the three species, a comparative study was carried out to investigate symptom progression, fungal systemic growth and translocation of DON following stem base inoculation of soft wheat. FCR symptoms were mainly localized in the inoculated area, which extended up to the second node for all inoculated species. Only the most aggressive strains caused symptoms up to the third node. Real‐time quantitative PCR showed that fungal colonization reached the third node for all the tested species, but a low percentage of plants showed colonization above the third node following inoculation with the most aggressive strains. Fungal growth was detected in symptomless tissues but none of the three species was able to colonize as far as the head tissues. However, even if the pathogens were not detected in the heads, DON was detected in head tissues of the plants inoculated with the most aggressive strains. These results demonstrate that F. pseudograminearum, F. culmorum and F. graminearum, under the same experimental conditions, follow a similar pattern of symptom progression, fungal colonization and DON translocation after stem base infection. Differences in the extent of symptoms, fungal colonization and mycotoxin distribution were mainly attributable to strain aggressiveness. These findings provide comparative information on the events following infection of the stem base of wheat by three of the most important FCR casual agents.