Phlebiopsis gigantea strains from Estonia show potential as native biocontrol agents against Heterobasidion root rot and contain diverse dsRNA and ssRNA viruses

Coniferous forests, especially Norway spruce (Picea abies L.) stands, are threatened by root rot disease caused by Heterobasidion annosum sensu lato (Fr.) Bref. The basidiomycetous fungus, Phlebiopsis gigantea (Fr.) Donk, is used as a biocontrol agent against this pathogen in several countries. In Estonia, stump treatments are currently conducted using the Rotstop preparation, which consists of a P. gigantea strain isolated in Finland. However, for ecological reasons it would be preferable to use an indigenous biocontrol strain. In this study, we tested new strains of P. gigantea isolated from Norway spruce stumps in Estonia for their growth rate and competitive ability against H. parviporum and H. annosum, and identified P. gigantea strains promising for biocontrol purposes. As viruses may cause phenotypic debilitation in fungi, we analyzed the virome of these strains by high-throughput sequencing (RNA-Seq) and assessed whether the presence of mycoviruses affects their performance as compared to virus-free strains of P. gigantea. Viruses related to members of Partitiviridae and Curvulaviridae, as well as unclassified ambi-like, aspi-like and “fusariviruses” were identified for the first time in P. gigantea.