Two viruses of Heterobasidion confer beneficial, cryptic or detrimental effects to their hosts in different situations

We investigated the effects of two recently described dsRNA mycoviruses, HetRV3-ec1 and HetRV6-ab6, on Heterobasidion wood decay fungi. The viruses originally inhabited Heterobasidion ecrustosum and Heterobasidion abietinum, and were transferred in the laboratory into other Heterobasidion species. Isogenic virus-free and virus-infected Heterobasidion isolates were used to test the effects of these viruses on their hosts' growth rate and competitive ability against mycorrhizal and decay fungi (Paxillus involutus, Meliniomyces bicolor and Phlebiopsis gigantea). This study shows that: (i) a single virus strain confers different effects on different Heterobasidion host strains; and (ii) a single virus strain may have contrasting effects on the fitness of a single host isolate (ranging from no effect to harmful or beneficial) depending on environmental and ecological conditions. We also report for the first time on the antagonism of Helotiales belonging to the sub-group Rhizocyphus-Meliniomyces against Heterobasidion species.