Population structure of a novel putative mycovirus infecting the conifer root-rot fungus Heterobasidion annosum sensu lato

► We describe a novel mycovirus from the root-rot fungus Heterobasidion annosum. ► The virus (HetRV6) was somewhat related to the Curvularia thermal tolerance virus. ► We found geographical and host-related differentiation among 35 virus strains. ► HetRV6 was transmitted in vitro between the different Heterobasidion species. ► HetRV6 seemed to be cryptic or slightly mutualistic to its host.