| Summary: | Some common ash trees (<i>Fraxinus excelsior</i>) show tolerance towards shoot dieback caused by the invasive ascomycete <i>Hymenoscyphus fraxineus</i>. Leaf petioles are considered to serve as a pathogen colonization route to the shoots. We compared four common ash clones with variation in disease tolerance, and included the native host, Manchurian ash (<i>Fraxinus mandshurica</i>), as a reference. Tissue colonization, following rachis inoculation by <i>H. fraxineus</i>, was monitored by histochemical observations and a quantitative polymerase chain reaction (qPCR) assay specific to <i>H. fraxineus</i>. Axial spread of the pathogen towards the petiole base occurred primarily within the phloem and parenchyma, tissues rich in starch in healthy petioles. In inoculated petioles, a high content of phenolics surrounded the hyphae, presumably a host defense response. There was a relationship between field performance and susceptibility to leaf infection in three of the four studied common ash clones, i.e., good field performance was associated with a low petiole colonization level and vice versa. Low susceptibility to leaf infection may counteract leaf-to-shoot spread of the pathogen in common ash, but the limited number of clones studied warrants caution and a larger study. The Manchurian ash clone had the highest petiole colonization level, which may suggest that this native host has evolved additional mechanisms to avoid shoot infection.
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