Intraspecific variation in host susceptibility and climatic factors mediate epidemics of sudden oak death in western US forests

Umbellularia californica is one of the key infectious hosts of the exotic Phytophthora ramorum, which causes sudden oak death (SOD) in California and Oregon forests. This study provides a comprehensive analysis of the epidemiologically relevant parameters for SOD in California and southern Oregon, including potential differences between the two states. Experimental infection of U. californica leaves was optimal when leaves were wet for 6–12 h, temperature was approximately 19°C and pathogen concentration was approximately 2·7 × 10⁴ zoospores mL^?1. Seasonal variation in host susceptibility and disease incidence was examined for two populations by inoculating detached leaves at 12 dates and by monitoring naturally infected leaves, respectively. Susceptibility of U. californica and disease incidence varied significantly in time and the variation was highest for both in spring. Susceptibility of trees from 17 natural populations from California and southern Oregon was assessed in detached leaf inoculations. One California and three southern Oregon populations had significantly and repeatable lower average susceptibility in artificial inoculations, but differences among three selected California and Oregon populations were not significant in inoculations of seedlings grown from seed in a common garden. This study concludes that U. californica susceptibility has a large environmental component, yet still predicts potential disease severity in different sites especially where infestations are young or the pathogen has not yet arrived. The accuracy and utility of predictive risk models for P. ramorum will be enhanced by the inclusion of both the environmental and host susceptibility components.     

Infectivity and sporulation potential of Phytophthora kernoviae to select North American native plants

Phytophthora kernoviae exhibits comparable epidemiology to Phytophthora ramorum in invaded UK woodlands. Because both pathogens have an overlapping geographic range in the UK and often concurrently invade the same site, it is speculated that P. kernoviae may also invade North American (NA) forests threatened by P. ramorum, the cause of Sudden Oak Death. This paper addresses the susceptibility of select NA plants to P. kernoviae, including measures of disease incidence and severity on wounded and unwounded foliage. The potential for pathogen transmission and survival was investigated by assessing sporangia and oospore production in infected tissues. Detached leaves of Rhododendron macrophyllum, Rhododendron occidentale and Umbellularia californica, and excised roots of U. californica and R. occidentale were inoculated with P. kernoviae and percent lesion area was determined after 6 days. Leaves were then surface sterilized and misted to stimulate sporulation and after 24 h sporangia production was assessed. The incidence of symptomless infections and sporulation were recorded. All NA native plants tested were susceptible to P. kernoviae and supported sporangia production; roots of U. californica and R. occidentale were both susceptible to P. kernoviae and supported sporangia production. Oospore production was also observed in U. californica roots. The results highlight the vulnerability of select NA native plants to infection by P. kernoviae, suggest that symptomless infections may thwart pathogen detection, and underscore the importance of implementing a proactive and adaptive biosecurity plan.     

Expression of defence‐related genes in stems and leaves of resistant and susceptible field pea (Pisum sativum) during infection by Phoma koolunga

The differential expression of 13 defence‐related genes during Phoma koolunga infection of stems and leaves of susceptible versus resistant field pea (Pisum sativum) was determined using qRT‐PCR. Expression, in terms of relative mRNA level ratios, of genes encoding ferredoxin NADP oxidoreductase, 6a‐hydroxymaackiain methyltransferase (hmm6), chalcone synthase (PSCHS3) and ascorbate peroxidase in leaves and stems differed during 6–72 hours post‐inoculation (hpi) and reflected known host resistance levels in leaves versus stems. In comparison to the susceptible genotype, at 24, 48 and 72 hpi, two genes, hmm6 (122.43‐, 206.99‐ and 32.25‐fold, respectively) and PSCHS3 (175.00‐, 250.13‐ and 216.24‐fold, respectively), were strongly up‐regulated in leaves of the resistant genotype, highlighting that resistance against P. koolunga in field pea is governed by the early synthesis of pisatin. At 24 hpi, leaves infected by P. koolunga showed clear differences in expression of target genes. For example, the gene encoding a precursor of the defensin ‘disease resistance response protein 39’ was substantially down‐regulated in leaves of both the susceptible and the resistant genotypes inoculated with P. koolunga. This contrasts with other studies on another pea black spot pathogen, Didymella pinodes, where this same gene is strongly up‐regulated in leaves of resistant and susceptible genotypes. The current study provides the first understanding of defence‐related genes involved in the resistance against P. koolunga, opening novel avenues to engineer new field pea cultivars with improved leaf and stem black spot disease resistance as the basis for developing more effective and sustainable management strategies.     

In-vitro characterisation of the four lineages of <Emphasis Type="Italic">Phytophthora ramorum</Emphasis>

Phytophthora ramorum is a plant pathogen with a wide host range including many ornamental hosts and tree species. In Ireland and the UK P. ramorum is known to cause sudden larch death. There are four distinct genetic lineages of P. ramorum, with the fourth lineage (EU2) described in 2012 and only present in Northern Ireland and Scotland. In this work, experiments that compare all four lineages of P. ramorum using several phenotypic characters are described. A total of 166 isolates (EU1: 116, EU2: 40, NA1: 8, NA2: 2) from several EU countries and the United States and several hosts were amassed, and a selection of isolates were compared according to standard phenotypic tests. The EU1 and EU2 isolates tested were all A1 mating type. Regarding linear growth rate, we found the isolates ranked as follows EU2 > NA2 > EU1 > NA1, with all lineages growing fastest at 20 °C. The lineages ranked as NA2 > EU1 > EU2 > NA1 based on their in-vitro aggressiveness on detached wounded Rhododendron leaves, all lineages most aggressive at 20 °C. At 20 °C, we found that there was no significant difference between the EU1 and EU2 lineage based on their linear growth rate or in-vitro aggressiveness. Temperature, host ramet and P. ramorum lineage all had statistically significant effects on the observed aggressiveness of the isolates. From an experimental point of view, our results are broadly in agreement with other phenotypic studies of P. ramorum, finding variation between the lineages, but also variation within the lineages. From an applied perspective, our work on Rhododendron indicates that the EU1 and EU2 lineages pose similar levels of threat to plant health in Ireland and the UK, however, how these results transfer to other hosts (e.g. Larix kaempferi) needs more study.     

Genetic diversity,sensitivity to phenylamide fungicides and aggressiveness of Phytophthora ramorum on Camellia,Rhododendron and Viburnum plants in Spain

Phytophthora ramorum has been detected in official plant health surveys on Rhododendron, Viburnum and Camellia in ornamental nurseries in northern Spain since 2003. A collection of 94 isolates of P. ramorum was obtained from 2003 to 2008 from plants with symptoms at different geographical locations. Isolates were identified based on morphology and sequence of the rDNA ITS region. Mating type, genetic variation, sensitivity to phenylamide fungicides and aggressiveness of these isolates were determined. All isolates belonged to the A1 mating type, ruling out the possibility of genetic recombination. Seven microsatellite markers were used to study genetic diversity; three out of the seven microsatellite markers were polymorphic within the Spanish population of P. ramorum. This study confirms that all Spanish isolates of P. ramorum belonged to the EU1 lineage. Twelve intralineage genotypes were detected, five that are unique to Spain (EU1MG38, EU1MG41, EU1MG37, EU1MG39 and EU1MG40) and seven that are also present in at least one other European country (EU1MG1, EU1MG29, EU1MG22, EU1MG13, EU1MG2, EU1MG18 and EU1MG26). Genotypes EU1MG37, EU1MG39 and EU1MG40 were isolated from Rhododendron from one region; EU1MG38 and EU1MG41 were isolated from Camellia from two different regions. Isolates of genotype EU1MG38 were resistant to metalaxyl and mefenoxam. The level of genetic diversity within the Spanish population of P. ramorum is limited and indicates a relatively recent clonal expansion.     

Spectral signatures in the UV range can be combined with secondary plant metabolites by deep learning to characterize barley–powdery mildew interaction

In recent studies, the potential of hyperspectral sensors for the analysis of plant–pathogen interactions was expanded to the ultraviolet range (UV; 200–380 nm) to monitor stress processes in plants. A hyperspectral imaging set-up was established to highlight the influence of early plant–pathogen interactions on secondary plant metabolites. In this study, the plant–pathogen interactions of three different barley lines inoculated with Blumeria graminis f. sp. hordei (Bgh, powdery mildew) were investigated. One susceptible genotype (cv. Ingrid, wild type) and two resistant genotypes (Pallas 01, Mla1- and Mla12-based resistance and Pallas 22, mlo5-based resistance) were used. During the first 5 days after inoculation (dai) the plant reflectance patterns were recorded and plant metabolites relevant in host–pathogen interactions were studied in parallel. Hyperspectral measurements in the UV range revealed that a differentiation between barley genotypes inoculated with Bgh is possible, and distinct reflectance patterns were recorded for each genotype. The extracted and analysed pigments and flavonoids correlated with the spectral data recorded. A classification of noninoculated and inoculated samples with deep learning revealed that a high performance can be achieved with self-attention networks. The subsequent feature importance identified wavelengths as the most important for the classification, and these were linked to pigments and flavonoids. Hyperspectral imaging in the UV range allows the characterization of different resistance reactions, can be linked to changes in secondary plant metabolites, and has the advantage of being a non-invasive method. It therefore enables a greater understanding of plant reactions to biotic stress, as well as resistance reactions.     

Potential susceptibility of Australian native plant species to branch dieback and bole canker diseases caused by Phytophthora ramorum

Susceptibility to branch dieback caused by Phytophthora ramorum was tested using a detached branch assay for 66 Australian native plant species sourced from established gardens and arboreta in California. Six of these species were further tested for their susceptibility to bole cankers caused by P. ramorum using a sealed log assay. Isopogon formosus and Eucalyptus denticulata were identified as potentially highly susceptible Australian branch dieback hosts. Thirteen potentially tolerant Australian host species included Banksia attenuata, B. marginata, E. haemastoma, E. regnans, Pittosporum undulatum and Billardiera heterophylla. Eucalyptus regnans was identified as a potentially highly susceptible bole canker host, while E. diversicolor and E. viminalis were considered potentially tolerant species to bole cankers caused by P. ramorum. Phytophthora ramorum was able to infect all 66 species, as confirmed by reisolation. These results extend the known potential host range for P. ramorum, confirm it as a possible threat to Australian plant industries and ecosystems and highlight additional associated hosts that are important in the global horticultural trade, native forests and plantation forestry.     

Effect of host factors on the susceptibility of Rhododendron to Phytophthora ramorum

Phytophthora ramorum causes sudden oak death (SOD) in western coastal forests of the USA. In Europe, the pathogen is mainly present in the nursery industry, particularly on Rhododendron. Because of the primary role of Rhododendron as a host and potentially as a vector, the effect of Rhododendron host factors on P. ramorum susceptibility and sporulation was investigated. Inoculation methods using either wounded or non‐wounded detached leaves were applied to 59 Rhododendron cultivars and 22 botanical species, replicated in three separate years. All Rhododendron species and cultivars were susceptible when using wounded leaves, but not when using non‐wounded leaves, suggesting a resistance mechanism operating at the level of leaf penetration. Using a regression tree analysis, the cultivars and species were split into four susceptibility classes. Young leaves were more susceptible than mature leaves when wounded, but less susceptible when non‐wounded. This effect was not correlated with leaf hydrophobicity or the number of leaf hairs. The presence or the type of rootstock did not affect the cultivar susceptibility level. Sporangia and chlamydospore production in the leaf lesions varied widely among Rhododendron cultivars and was not correlated with the susceptibility level. The susceptibility to P. ramorum correlated well with the susceptibility to P. citricola and P. hedraiandra × cactorum, suggesting that the resistance mechanisms against these species are non‐specific. Susceptibility to P. kernoviae was low for most cultivars. These findings have implications for detection, spread and disease control, and are therefore important in pest risk assessment.     

Characterizing the variation in aggressiveness and sporulation of the NA1 and EU1 lineages of Phytophthora ramorum in Oregon

Phytophthora ramorum, the cause of sudden oak death, is an invasive pathogen present in parts of coastal California and south-western Oregon forests. The majority of these forest infestations have been caused by the NA1 clonal lineage. In 2015, the EU1 lineage of P. ramorum was isolated from a tanoak (Notholithocarpus densiflorus) tree located in a mixed-conifer forest of Curry County, Oregon. In order to evaluate the threat to Oregon forests of the EU1 lineage relative to the established NA1 lineage, a series of experiments was conducted comparing aggressiveness and sporulation of NA1 and EU1 isolates on logs and seedlings in the growth chamber and forest. There was no significant difference in lesion size on logs inoculated with NA1 and EU1 isolates for any of the tree species tested. Across all seedling experiments differences among isolates within lineage, in terms of both aggressiveness and sporulation, were more commonly observed than differences among lineages. Site to site variation in tanoak sporulation, as measured by rain bucket baiting, appears to be correlated with the number of P. ramorum-positive seedlings detected at each site.     

Coincidence of maximum severity of powdery mildew on grape leaves and the carbohydrate sink‐to‐source transition

Grapevine leaves infected with powdery mildew are a source of inoculum for fruit infection. Leaves emerging on a single primary shoot of Vitis vinifera cv. Cabernet Sauvignon were exposed to average glasshouse temperatures of 18°C (0·23 leaves emerging/day) or 25°C (0·54 leaves emerging/day). All leaves on 8–10 shoots with approximately 20 leaves each were inoculated with Erysiphe necator conidia to assess disease severity after 14 days in the 25°C glasshouse. Two photosynthetic ‘source’ leaves per shoot on the remaining 8–10 shoots were treated with ¹⁴CO₂ to identify, by autoradiography, the leaf position completing the carbohydrate sink‐to‐source transition. There was a clear association between the mean modal leaf position for maximum severity of powdery mildew (position 3·7 for 18°C; position 4·4 for 25°C) and the mean position of the leaf completing the sink‐to‐source transition (position 3·8 for 18°C; position 4·7 for 25°C). The mean modal leaf position for the maximum percentage of conidia germinating to form secondary hyphae was 4·2 for additional plants grown in the 25°C glasshouse. A higher rate of leaf emergence resulted in a greater proportion of diseased leaves per shoot. A Bayesian model, consisting of component models for disease severity and leaf ontogenic resistance, had parameters representing the rate and magnitude of pathogen colonization that differed for shoots developing in different preinoculation environments. The results support the hypothesis that the population of leaves in a vineyard capable of supporting substantial pathogen colonization will vary according to conditions for shoot development.     

The role of aggressiveness and competition in the selection of Phytophthora infestans populations

Selection within populations of Phytophthora infestans was investigated by comparing the aggressiveness of single‐lesion isolates on detached leaflets of four potato cultivars with differing levels of race‐nonspecific resistance to P. infestans. The isolates included 23 representative of Northern Ireland genotypes from the early 2000s, used to inoculate previously reported field trials on competitive selection (2003–2005), plus 12 isolates recovered from the 2003 trial. The cultivars were those planted in the previous trials: Atlantic (blight‐susceptible) and Santé, Milagro and Stirling (partially resistant). Very highly significant variation for latent period, infection frequency and lesion area was found between genotypes and cultivars; differences between genotypes were more marked on the more resistant cultivars, but no one genotype was the most aggressive across all. Detached leaflets were also inoculated with mixtures of isolates from each genotype group at three sporangial concentrations: differences in aggressiveness between genotypes were more apparent at lower concentrations and on the more resistant cultivars. Genotype groups that were the most aggressive on the more resistant cultivars tended to be those selected by the same cultivars in the field. A mixture of all isolates of all genotypes was used to inoculate detached leaflets of the same cultivars. With one exception, single spore isolates recovered from any one leaflet belonged to a single genotype, but different genotypes were recovered from different cultivars. Phytophthora infestans isolates from Northern Ireland showed significant variation for foliar aggressiveness, and pathogen genotypes exhibited differential aggressiveness to partially resistant cultivars and interacted competitively in genotype selection.     

Geographic location and year determine virulence,and year determines genetic change,in populations of Neopseudocercosporella capsellae

White leaf spot (Neopseudocercosporella capsellae) can be severe and problematic worldwide across both horticultural and oilseed Brassicaceae, including susceptible rapeseed. In this study, 82 isolates from 2015 and 106 isolates from across Australia in 2016 were first assessed for their virulence against three different rapeseed (Brassica napus) cultivars. For both years there were significant (P < 0.001) differences. Also, there were significant (all P < 0.001) differences between isolates in each year, and between cultivars. For 2016 isolates, there were also significant differences (P < 0.001) between isolates across three different Australian states, and a significant interaction (P < 0.001) between isolates with cultivars. Of the three Australian states, isolates from Victoria were most virulent. Among tested cultivars, cv. Scoop was most susceptible. Subsequently, phylogenetic analysis of 114 of these same 2015 and 2016 isolates showed current isolates clustered separately from the majority of 2005 N. capsellae isolates collected from Western Australia a decade earlier, confirming significant genetic change within N. capsellae populations over the past decade. However, isolate clusters showed no association with geographical location. The results suggest that phylogenetic association among 2005 and 2015–2016 N. capsellae isolates is complementary with pathogenicity variations explained by geographically different N. capsellae pathogen populations. Neopseudocercosporella capsellae populations are evolving rapidly, challenging management through host resistance at a time of increasing incidence and severity of white leaf spot disease over the past decade. The outcome is well illustrated by cv. Scoop, previously resistant to 2005 isolates but moderately susceptible to 2015 and highly susceptible to 2016 isolates.     

Quantifying resistance to Plasmodiophora brassicae in Brassica hosts

Plasmodiophora brassicae causes clubroot of crucifers. A quantitative PCR (qPCR)‐based protocol was developed to measure P. brassicae DNA in the roots of susceptible, intermediately susceptible, intermediately resistant and resistant Brassica hosts, and the non‐host wheat, at 5, 10, 15, 20 and 42 days post‐inoculation (dpi). The final reaction of each plant genotype was recorded as an index of disease at 42 dpi. Plasmodiophora brassicae DNA showed an increase in susceptible and moderately resistant hosts from 5 to 42 dpi, in contrast to a decrease in a highly resistant host and the non‐host wheat over the same period. Index of disease was significantly positively correlated with the amount of P. brassicae DNA in the roots at 5, 15, 20 and 42 dpi in one experiment, and at 10, 15, 20 and 42 dpi in a repeated experiment. Significant positive correlations also existed between the amounts of P. brassicae DNA in the roots at 42 dpi and those at 5, 10, 15 and 20 dpi in one experiment, and those at 10, 15 and 20 dpi in a repeated experiment. The results generated by the qPCR assay were validated by microscopic examination of roots inoculated with P. brassicae. The qPCR‐based protocol developed in this study allows for the accurate quantification of P. brassicae DNA in host root tissues as early as 5 dpi, and may serve as a useful tool to evaluate pathogen proliferation and development in the roots.     

Zu den Ausbreitungswegen von Phytophthora ramorum an Rhododendron und Viburnum auf Container-Stellflächen

In Europe, Phytophthora ramorum basically infects Rhododendron and Viburnum. To prevent the spread of the new quarantine organism in nurseries more knowledge about the transmission biology of this pathogen is necessary. For this reason the pathways of spread for P. ramorum on the two main host plants have been studied for the first time. Under practical field conditions inoculated plants were placed as sources of infections in a larger stock. Over 3?months the development of infestation was recorded. The pathogen showed a poor potential of spread. At the end of the trial only 1.0% of Rhododendron and 0.3% of Viburnum were infested with P. ramorum. Typical symptoms could be observed. On Rhododendron the pathogen caused a branch dieback. Stems showed a brown discoloration, which starts usually at the tip of the twig and moved towards the base. Infected Viburnum showed a stem base rot with wilting symptoms. Additionally rhododendrons were natural infested with Phytophthora citricola. This pathogen caused the same symptoms like P. ramorum and spread much faster. Investigations of leaf litter showed that both Phytophthora species had colonized the ground. This observation and the pattern of spread indicate that inoculum on the ground has been transmitted to arial plant parts via rainsplash. There is little evidence that P. ramorum has been transmitted directly from plant to plant via splashwater or air.     

Phytophthora kernoviae and P. ramorum: host susceptibility and sporulation potential on foliage of susceptible trees1

Phytophthora kernoviae and P. ramorum are introduced, invasive pathogens in the UK. Both species are adapted for aerial dispersion and have a wide host range, many of which are common to both pathogens. The diseases they cause are foliar necrosis and shoot tip dieback on both tree and ornamental hosts, and bleeding cankers on tree hosts. Inoculum is produced on infected foliage but not on bleeding cankers in both cases. Proactive measures to prevent disease spread and to evaluate the risks posed by these pathogens are being undertaken. Amongst others, these include using the detached leaf assay to get an indication of tree foliage susceptibility, and inoculating wounded stems of saplings to get an idea of under‐canopy sapling and nursery tree susceptibility. The sporulation potential on selected susceptible hosts was assessed, and finally, surveys which are still ongoing were carried out in south‐west England. Results of this work are presented and discussed.     

Selection on Erysiphe graminis in pure and mixed stands of barley

The response of populations of Erysiphe graminis f. sp. hordei to selection by pure and mixed stands of three spring barley cultivars was studied in two field trials. The range of virulence of the pathogen genotypes selected in mixed host stands was dependent on the relative fitness of each genotype over all hosts. Unnecessary virulences were rapidly selected against on some hosts, but were less deleterious or favoured on others. In general there was selection for widely adapted pathogen genotypes in mixed host populations but this selection for flexibility limited the abilitytions in the absolute size of the pathogen population in host mixtures reduced the absolute frequencies of pathogen genotypes with combined virulences in comparison with those in pure stands. It is argued that host mixtures are therefore unlikely to favour rapid pathogen evolution towards races which are both widely adapted and highly virulent on all component cultivars which they can infect his definition docs not conform with conventional usage in population genetics.     

Molecular phylogenetic,morphological, and pathogenic analyses reveal a single clonal population of Septoria lycopersici with a narrower host range in Brazil

Septoria leaf spot, caused by Septoria lycopersici, is considered one of the most important diseases of tomato in Brazil. Despite its importance, the disease agent is still poorly studied. Septoria isolates collected from different production regions of Brazil were characterized by molecular, morphological, and pathogenic methods. A set of 104 isolates was sequenced for the DNA Tub, Cal, and EF1-α loci. Ten isolates were selected, according to geographical region of origin and type of leaf lesion (typical or atypical), for morphological characterization and for evaluation of aggressiveness on tomato cultivar Santa Clara. To evaluate the pathogen host range, cultivated and wild Solanaceae plants were inoculated with four selected isolates. The results showed that all isolates grouped with the type isolate of S. lycopersici in maximum likelihood and Bayesian inference trees. The isolates were morphologically similar. All isolates selected for pathogenicity testing on tomato were able to induce typical symptoms of the disease, but differed in their aggressiveness. A total of eight species of Solanaceae were also identified as potential alternative hosts for S. lycopersici. This information will provide a more accurate assessment of the risks involved with the introduction of new crops, especially of the genus Solanum, in areas where the species is already present. In addition, it will provide the basis for the establishment of more efficient methods in the management of Septoria leaf spot of tomatoes in natural conditions and in the different production systems.     

Exploring de novo specificity: the Pyrenophora tritici‐repentis–barley interaction

Pyrenophora tritici‐repentis (Ptr) is a destructive fungal pathogen of wheat worldwide. In addition to wheat, Ptr has been isolated from various other hosts in the family Poaceae, yet the nature of its interaction with those hosts is unknown. The Ptr–barley relationship was explored and the existence of a specific interaction between Ptr and barley is described for the first time; symptom development on several barley genotypes was evaluated in bioassays and by toxin infiltration into barley leaves. Ptr ToxB‐producing isolates of the fungus were able to cause significant damage when inoculated onto certain barley genotypes, and Ptr ToxB was able to induce chlorosis in a highly selective manner when infiltrated into those same genotypes. Ptr–barley specificity is subtle and can break with slight changes in temperature after infection. To understand the infection process in barley, a cytological analysis and in planta fungal biomass estimation using quantitative PCR were performed. The fungus penetrates through the host epidermal cells and advances to colonize the mesophyll layer intercellularly, with the infection process on barley closely resembling that on wheat. Here, evidence is provided for a specific interaction between barley and Ptr, expanding understanding of Ptr host specificity and breaking the assumption that the highest level of specificity seen with Ptr is restricted to particular genotypes of the wheat host.     

Phenotypic and molecular genetic characterization indicate no major race‐specific interactions between Xanthomonas translucens pv. graminis and Lolium multiflorum

Bacterial wilt of forage grasses, caused by the pathogen Xanthomonas translucens pv. graminis (Xtg), is a major disease of forage grasses such as Italian ryegrass (Lolium multiflorum). The plant genotype‐bacterial isolate interaction was analysed to elucidate the existence of race‐specific responses and to assist the identification of plant disease resistance genes. In a greenhouse experiment, 62 selected plant genotypes were artificially inoculated with six different bacterial isolates. Significant differences in resistance were observed among L. multiflorum genotypes (P < 0·001) and in virulence (intensity of disease symptoms) among Xtg isolates (P < 0·001) using the area under the disease progress curve (AUDPC). No significant genotype‐isolate interaction (P > 0·05) could be observed using linear regression modelling. However, additive main effects and multiplicative interaction effects (ammi ) analysis revealed five genotypes which did not cluster close to the origin of the biplot, indicating specific interactions between these genotypes and some bacterial isolates. Simple sequence repeat (SSR) markers were used to identify marker‐resistance associations using the same plant genotypes and bacterial isolates. The SSR marker NFA027 located on linkage group (LG) 5 was significantly associated with bacterial wilt resistance across all six bacterial isolates and explained up to 37·4% of the total variance of AUDPC values. Neither the inoculation experiment nor the SSR analyses revealed major host genotype‐pathogen isolate interactions, thus suggesting that Xtg resistance, observed so far, is effective across a broad range of different bacterial isolates and plant genotypes.