Abstract
Some common ash trees (Fraxinus excelsior) show tolerance towards shoot dieback caused by the invasive ascomycete Hymenoscyphus fraxineus. 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 (Fraxinus mandshurica), as a reference. Tissue colonization, following rachis inoculation by H. fraxineus, was monitored by histochemical observations and a quantitative polymerase chain reaction (qPCR) assay specific to H. fraxineus. 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.
Highlights
Since the 1990s, common ash forests have become increasingly damaged because of the rapid spread of an invasive ascomycete, which causes shoot dieback and mortality of ash species native to Europe
We tested our hypothesis using a H. fraxineus-specific real-time PCR assay to monitor the progress of H. fraxineus colonization and by registering cytological host defense responses in leaf petioles of F. excelsior clones, with variation in susceptibility to shoot dieback and with the native host Manchurian ash serving as a reference
The common ash genotypes originated from two trials, the clonal trial “Tapsøre” (FP281) established in 1998 and located at 55◦ 240 14.2” N 9◦ 270 27.5” E [13], and the progeny trial “Randers” (F384) established in 2004 and located at 56◦ 300 00.0” N
Summary
Since the 1990s, common ash forests have become increasingly damaged because of the rapid spread of an invasive ascomycete, which causes shoot dieback and mortality of ash species native to Europe. In European arboreta, the pathogen seems to effectively infect leaf tissues of several native Asian host species, as well as the European ones, the difference being that no or only minor shoot symptoms are induced in Asian trees [5]. As the petiole-to-shoot route is considered to be the main path for shoot infection by H. fraxineus, we hypothesized that the reduced spread of the pathogen and more pronounced defense responses would take place in the leaf tissues of common ash genotypes with a strong field performance, i.e., trees with a low level of shoot dieback in field locations with an epidemic level of the disease. We tested our hypothesis using a H. fraxineus-specific real-time PCR assay to monitor the progress of H. fraxineus colonization and by registering cytological host defense responses in leaf petioles of F. excelsior clones, with variation in susceptibility to shoot dieback and with the native host Manchurian ash serving as a reference
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