Hymenoscyphus linearis sp nov: another close relative of the ash dieback pathogen H. fraxineus

Abstract

Hymenoscyphus is a large fungal genus containing a single known severe pathogen (H. fraxineus) causing ash dieback in Europe. Here, the closely related species Hymenoscyphus linears sp. nov. is described from Japan. Apothecia of this fungus emerge from linear, black pseudosclerotia on rachises and petioles of Fraxinus platypoda. In culture, the species forms a Chalara anamorph similar to that of H. fraxineus. In addition, a Sporotrichum-like synanamorph is produced. Spores of neither anamorphic forms germinate on malt extract agar and might act solely as spermatia. After prolonged incubation, ascocarps are produced directly on agar plates, indicating the ability to self-fertilize. Phylogenetic investigations using four sequence markers placed the species alongside H. albidus, H. albidoides, and H. fraxineus. Eight fungal strains, co-occurring with H. linearis on petioles of F. platypoda, were isolated and identified by sequencing the ITS region of the rDNA. Seven different species were revealed, showing that there is no other dominant fungus on petioles of F. platypoda. Pathogenicity tests on Fraxinus excelsior and its native host F. platypoda revealed that the fungus is avirulent. The close phylogenetic relationship with H. fraxineus and the avirulence of H. linearis offer an opportunity to study the evolution of pathogenicity of H. fraxineus in further detail in the future.