Evaluation and identification of viruses for biocontrol of the ash dieback disease

The invasive ascomycete Hymenoscyphus fraxineus is the causative agent for ash dieback on the European species Fraxinus excelsior and Fraxinus angustifolia, and there is concern that it is going to replace the native, closely related and nonpathogenic Hymenoscyphus albidus. Fungal management in forests is limited, and alternative approaches for control are needed. Within the scope of the project “FraxForFuture”, several strategies are being investigated. One idea comprises the use of a viral hyperparasite, which can induce a reduced virulence in the fungal host H. fraxineus in an antagonist-like system. This phenomenon, the reduction of fungal virulence by a viral infection, is known as hypovirulence, and a similar method has already been established to control the Chestnut Blight in Europe. We examined 34 isolates of H. fraxineus for both their virulence and presence of a viral infection. Although a predominant number of isolates were found to be infected with Hymenoscyphus mitovirus 1 (HfMV1), no additional viruses were detected, and our data did not indicate a link to reduced virulence. The search for a viral infection was extended to one isolate of H. albidus in which we found and characterized a novel mycovirus. Based on phylogenetic analysis and sequence properties, it was assigned to the genus Victorivirus in the family of Totiviridae and was tentatively denominated as Hymenoscyphus albidus victorivirus 1. This novel and native mycovirus might be suitable for inducing hypovirulence in H. fraxineus as a biocide.