Direct evidence of Hymenoscyphus fraxineus infection pathway through the petiole‐shoot junction

Abstract

The symptoms of ash dieback caused by the fungus Hymenoscyphus fraxineus include wilting of the foliage followed by dieback of shoots, twigs and branches. Necroses in shoots are assumed to develop after infection through leaf petioles; however, clear evidence of this infection pathway has not yet been provided. Considering the multiple pathogen genotypes in dead ash petioles, we aimed to obtain a spatial overview of all H. fraxineus genotypes colonizing individual shoots and their corresponding petioles before leaf shedding to acquire precise information about the infection biology of H. fraxineus and its ability to cross the petiole-shoot junction. Individual genotypes of H. fraxineus were characterized by the analysis of microsatellites using DNA extracted directly from petiole segments or cultures isolated from the segments. We detected 150 different multilocus genotypes in 10 analysed shoots and their respective petioles; the highest number of genotypes was eight for a single petiole and three for a single shoot. The genotypes of most shoot lesions were identical to particular genotypes from the proximal segments of petioles, implicating the main pathway of shoot infections. To test whether the amount of colonized substrate or intraspecific competition have an effect on successful infection, genotypes that reached the most proximal end of the petioles were scored for the number of invaded petiole segments and for the number of other H. fraxineus genotypes co-occurring in the segments. However, the extent of colonization of the scored genotypes and intraspecific competition with other H. fraxineus strains did not influence pathogen success in entering the shoot.This study confirms that the majority of ash shoot infections are caused by genotypes of H. fraxineus originating from petioles. Compared to petioles, the frequency of shoot colonization as well as number of H. fraxineus genotypes in shoots was much lower.

Analyzing Ash Leaf-Colonizing Fungal Communities for Their Biological Control of Hymenoscyphus fraxineus

“…To identify fungal taxa associated with tolerance to H. fraxineus, we analyzed the fungal communities of ash leaves by ITS2 amplicon sequencing and culturing. Leaf-colonizing fungi are considered as a barrier to invaders such as H. fraxineus that spread starting from leaf infections (Cross et al, 2017;Hanackova et al, 2017b). The indigenous fungi fight against the invading pathogen directly by resource and space competition, partly supported by antibiosis, parasitism or indirectly by the induction of plant resistance (Busby et al, 2016;Jia et al, 2020).…”

The Endophytic Mycobiome of European Ash and Sycamore Maple Leaves – Geographic Patterns, Host Specificity and Influence of Ash Dieback

“…Most shoot infections by the ash dieback pathogen are caused by genotypes that originated from leaves and crossed the petiole-shoot junction ( Haňáčková et al, 2017b ). Although direct entrance to the petiole is possible ( Cleary et al, 2013 ), more infections are expected to happen in leaflets due to the much larger surface area.…”

An overview of ash ( Fraxinus spp.) and the ash dieback disease in Europe.

“…Leaves and petioles are infected by airborne ascospores that can be transported over long distances [5,7]. The fungus can spread to woody tissues through the petiole-shoot junction [9], where it causes bark necroses and locally kills shoots and twigs. The successive crown decline caused by every year occurring infections often is lethal to the trees.…”