Armillaria root rot fungi host single-stranded RNA viruses

<jats:title>Abstract</jats:title><jats:p>Species of <jats:italic>Armillaria</jats:italic> are distributed globally and include some of the most important pathogens of forest and ornamental trees. Some of them form large long-living clones that are considered as one of the largest organisms on earth and are capable of long-range spore-mediated transfer as well as vegetative spread by drought-resistant hyphal cords called rhizomorphs. However, the virus community infecting these species has remained unknown. In this study we used dsRNA screening and high-throughput sequencing to search for possible virus infections in a collection of <jats:italic>Armillaria</jats:italic> isolates representing three different species: <jats:italic>Armillaria mellea</jats:italic> from South Africa<jats:italic>, A. borealis</jats:italic> from Finland and Russia (Siberia) and <jats:italic>A. cepistipes</jats:italic> from Finland. Our analysis revealed the presence of both negative-sense RNA viruses and positive-sense RNA viruses, while no dsRNA viruses were detected. The viruses included putative new members of virus families <jats:italic>Mymonaviridae</jats:italic>, <jats:italic>Botourmiaviridae</jats:italic> and <jats:italic>Virgaviridae</jats:italic> and members of a recently discovered virus group tentatively named “ambiviruses” with ambisense bicistronic genomic organization. We demonstrated that <jats:italic>Armillaria</jats:italic> isolates can be cured of viruses by thermal treatment, which enables the examination of virus effects on host growth and phenotype using isogenic virus-infected and virus-free strains.</jats:p>