The fungal genus includes some of the most devastating conifer pathogens in the boreal forest region. In this study, we showed that the alphapartitivirus Heterobasidion partitivirus 13 from (HetPV13-an1) is the main causal agent of severe phenotypic debilitation in the host fungus. Based on RNA sequencing using isogenic virus-infected and cured fungal strains, HetPV13-an1 affected the transcription of 683 genes, of which 60% were downregulated and 40% upregulated. Alterations observed in carbohydrate and amino acid metabolism suggest that the virus causes a state of starvation, which is compensated for by alternative synthesis routes. We used dual cultures to transmit HetPV13-an1 into new strains of and The three strains of that acquired the virus showed noticeable growth reduction on rich culturing medium, while only two of six isolates tested showed significant debilitation. Based on reverse transcription-quantitative PCR (RT-qPCR) analysis, the response toward HetPV13-an1 infection was somewhat different in and We assessed the effects of HetPV13-an1 on the wood colonization efficacy of in a field experiment where 46 Norway spruce trees were inoculated with isogenic strains with or without the virus. The virus-infected strain showed considerably less growth within living trees than the isolate without HetPV13-an1, indicating that the virus also causes growth debilitation in natural substrates. A biocontrol method restricting the spread of species would be highly beneficial to forestry, as these fungi are difficult to eradicate from diseased forest stands and cause approximate annual losses of €800 million in Europe. We used virus curing and reintroduction experiments and RNA sequencing to show that the alphapartitivirus HetPV13-an1 affects many basic cellular functions of the white rot wood decay fungus, which results in aberrant hyphal morphology and a low growth rate. Dual fungal cultures were used to introduce HetPV13-an1 into a new host species, , and field experiments confirmed the capability of the virus to reduce the growth of in living spruce wood. Taken together, our results suggest that HetPV13-an1 shows potential for the development of a future biocontrol agent against fungi.
