Detection and genetic characterisation of a novel mycovirus in Hymenoscyphus fraxineus, the causal agent of ash dieback.

The ascomycete fungus Hymenoscyphus fraxineus (synonym: Hymenoscyphuspseudoalbidus, basionym: Chalara fraxinea) is a new invasive pathogen causing severe dieback of ash trees (Fraxinus spp.) in Europe. The disease was first recorded in the 1990s in Eastern Poland and Lithuania and has meanwhile spread across the continent and to Great Britain. Mycoviruses are commonly found in all major groups of plant pathogenic fungi. Some of these viruses can cause debilitating disease or reduce virulence in their fungal host and thereby can act as biological control agents (e.g. hypoviruses in the chestnut blight fungus). The main aim of our study is to identify mycoviruses in the ash dieback pathogen and to describe their phylogenetic position. Using RNAseq we could identify a viral sequence in two fungal isolates. Amino acid sequence comparison suggests that the detected mycovirus is a putative new member of the genus Mitovirus (family Narnaviridae), and we suggest naming it Hymenoscyphus fraxineus mitovirus 1 (HfMV1). The novel mitovirus is highly variable amongst European isolates and seems to diverge rapidly. This is very promising for the future search of a strain adequate for biological control measures.