Variation in aggressiveness of Hymenoscyphus fraxineus genotypes amid the ash dieback epidemic

Abstract

The ascomycete Hymenoscyphus fraxineus causes devastating damage to the European common ash (Fraxinus excelsior). The fungus originates from Asia, where it coexists with native ash species and completes its life cycle by sporulating on degrading ash leaf litter on the forest floor. Given this life cycle of the fungus, genotypes of H. fraxineus with varying degrees of aggressiveness may coexist in infected European ash forests. To test this hypothesis, we cultured 19 single‐spore isolates from apothecia collected in a trial of heavily infected ash in Denmark and carried out stem inoculations on young ash seedlings. Microsatellite markers revealed that the 19 isolates were all genetically unique and did not show any genetic structure. High variation was observed among the 19 isolates in pathogenicity on young seedlings. The variation (assessed as necrosis development) was highly significant, but not correlated with the vigour of the fungal isolates when grown on culture media. The results support the hypothesis that aggressiveness of H. fraxineus may not be crucial for its fitness. In this sense, ash dieback disease may differ from other recent emerging infectious diseases on trees. We discuss the variation in aggressiveness of H. fraxineus in relation to durability of resistance and future management of the ash dieback epidemic. The findings of this study call for more research into natural variation in endophytic versus pathogenic behaviour of H. fraxineus on European ash.

Assessment of lesion development in Fraxinus excelsior cultivars Altena, Atlas and Westhof's Glorie inoculated with different isolates of Hymenoscyphus fraxineus

“…In line with other studies, we found significant differences between isolates in lesion length (Kosawang et al, 2020;Kowalski & Holdenrieder, 2009;Lygis et al, 2017) and also in the percentage of inoculations resulting in lesions (Adamčíková et al, 2018). We found that the isolates CBS 145275 and CBS 122504 always induced lesions in both experiments.…”

Multiple, single trait GWAS and supervised machine learning reveal the genetic architecture ofFraxinus excelsiortolerance to ash dieback in Europe

“…Surpassing the tolerance offered by a single SNP would not make a large quantitative difference to the overall tree phenotype. Combined with recent results which present evidence that the biology of the pathogen does not favour selection of high virulence (Kosawang et al, 2020), and evidence of superior reproductive success of less susceptible individuals, gives some hope for the future of common ash (Semizer-Cuming et al, 2019. High mortality among the current generation is an acute concern but higher fitness of tolerant individuals, combined with restoration efforts and breeding programs will strongly support the continuation of F. excelsior in European forests.…”

Endophytic fungi related to the ash dieback causal agent encode signatures of pathogenicity on European ash

“…Parafilm was wrapped around the stem. A culture of H. fraxineus strain 18.3 (Kosawang et al ., 2020) was used as a positive control and sterile wood plugs as negative controls. Three biological replicates were performed for each inoculation type (i.e.…”

“…We address this question by studying three Hymenoscyphus species closely related to H. fraxineus : H. albidus , isolated from European ash growing in Wales (UK), H. koreanus , and H. occultus ; two newly-discovered species of Asian origin and not (yet) introduced to Europe; they have been isolated by Gross and Han (2015) from petioles of Korean ash ( F. chinensis subsp. rhynchophylla ) (Gross and Han, 2015; Kosawang et al ., 2020).…”