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.

Early molecular signatures of responses of wheat to Zymoseptoria tritici in compatible and incompatible interactions.

Zymoseptoria tritici, the causal agent of septoria tritici blotch, a serious foliar disease of wheat, is a necrotrophic pathogen that undergoes a long latent period. Emergence of insensitivity to fungicides, and pesticide reduction policies, mean there is a pressing need to understand septoria and control it through greater varietal resistance. Stb6 and Stb15, the most common qualitative resistance genes in modern wheat cultivars, determine specific resistance to avirulent fungal genotypes following a gene-for-gene relationship. This study investigated compatible and incompatible interactions of wheat with Z. tritici using eight combinations of cultivars and isolates, with the aim of identifying molecular responses that could be used as markers for disease resistance during the early, symptomless phase of colonization. The accumulation of TaMPK3 was estimated using western blotting, and the expression of genes implicated in gene-for-gene interactions of plants with a wide range of other pathogens was measured by qRT-PCR during the presymptomatic stages of infection. Production of TaMPK3 and expression of most of the genes responded to inoculation with Z. tritici but varied considerably between experimental replicates. However, there was no significant difference between compatible and incompatible interactions in any of the responses tested. These results demonstrate that the molecular biology of the gene-for-gene interaction between wheat and Zymoseptoria is unlike that in many other plant diseases, indicate that environmental conditions may strongly influence early responses of wheat to infection by Z. tritici, and emphasize the importance of including both compatible and incompatible interactions when investigating the biology of this complex pathosystem.