Species of the Heterobasidion annosum sensu lato (s. l.) complex are causing root and butt rot of conifers. Mycoviruses are usually cryptic, but some of them may cause hypovirulence (reduced virulence) or mutualistic effects on their fungal hosts. We explored new Heterobasidion viruses, and analyzed their taxonomy and effects on their hosts. The viruses were obtained from fungal culture collection of the Natural Resources Institute Finland (Luke) or from newly collected isolates.
This thesis addressed the taxonomy of Heterobasidion viruses as well as their transmission, effects on hosts’ phenotypes, and distribution. A new dsRNA virus from H. annosum s.s., Heterobasidion RNA virus 6 (HetRV6), was taxonomically distant from all previously known viruses of Heterobasidion spp., but related to the mutualistic Curvularia thermal tolerance virus. Populations of this species exhibited a considerable degree of geographical and host-related differentiation. Virus isolates HetRV6-ab6 and Heterobasidion partitivirus 3 (strain HetPV3-ec1) conferred different and condition-dependent effects on different host strains. Four new partitivirus species, HetPV12, HetPV13, HetPV14 and HetPV15, clustered in a clade within the genus Alphapartitivirus, that includes also HetPV3 and Helicobasidium mompa partitivirus V70. HetPV13 strains were found to have a high dispersal capacity. A high infection rate by four species of partitiviruses was observed in H. annosum in a heavily infected forest. Two of these species were previously unknown (HetPV16 and HetPV20). Three fungal isolates were co-infected by two different partitiviruses (HetPV13-an2 and HetPV7-an1 or HetPV16-an1 and HetPV20-an1), supporting the view that multiple infections are common.
Taken together, the global diversity and prevalence of Heterobasidion viruses is considerable, and their transmission may occur between somatically incompatible strains. They may co-infect single host strains, transmit over species borders and confer variable phenotypic effects on their hosts. Further studies are necessary to determine the biocontrol potential of these viruses.