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.

Eighty-eight Phytophthora cactorum strains isolated from crown or leather rot of strawberry in 1971-2019 were screened for viruses using RNA-seq and RT-PCR. Remarkably, all but one isolate were virus-infected, most of them harbouring more than one virus of different genera or species. The most common virus occurring in 94% of the isolates was the Phytophthora cactorum RNA virus 1 (PcRV1) resembling members of Totiviridae. Novel viruses related to members of Endornaviridae, named Phytophthora cactorum alphaendornaviruses 1-3 (PcAEV1-3), were found in 57% of the isolates. Four isolates hosted viruses with affinities to Bunyaviridae, named Phytophthora cactorum bunyaviruses 1-3 (PcBV1-3), and a virus resembling members of the proposed genus 'Ustivirus', named Phytophthora cactorum usti-like virus (PcUV1), was found in a single isolate. Most of the virus species were represented by several distinct strains sharing ≥81.4% aa sequence identity. We found no evidence of spatial differentiation but some temporal changes in the P. cactorum virus community were observed. Some isolates harboured two or more closely related strains of the same virus (PcAEV1 or PcRV1) sharing 86.6%-96.4% nt identity in their polymerase sequence. This was surprising as viruses with such a high similarity are typically mutually exclusive.

This report describes the complete genome sequence of a double-stranded RNA (dsRNA) virus infecting the oomycetous plant pathogen Phytophthora cactorum. The virus genome consists of a single dsRNA segment of 5699 bp with two open reading frames predicted to overlap with each other and encoding a putative capsid protein of 705 aa and an RNA-dependent RNA polymerase of 779 aa. Sequence comparisons indicated that this virus, designated as “Phytophthora cactorum RNA virus 1” (PcRV1), shares the highest sequence similarity with the unclassified Pythium splendens RNA virus 1 (58% RdRp aa sequence identity). Phylogenetic analysis revealed that these two oomycete viruses group together with Giardia lamblia virus (GVL; family Totiviridae) and several unclassified toti-like viruses from arthropods, fish and fungi. This is the first report of a toti-like virus in a member of the genus Phytophthora and the first virus characterized in P. cactorum.

Soils support a myriad of organisms hosting highly diverse viromes. In this minireview, we focus on viruses hosted by true fungi and oomycetes (members of Stamenopila, Chromalveolata) inhabiting bulk soil, rhizosphere and litter layer, and representing different ecological guilds, including fungal saprotrophs, mycorrhizal fungi, mutualistic endophytes and pathogens. Viruses infecting fungi and oomycetes are characterized by persistent intracellular nonlytic lifestyles and transmission via spores and/or hyphal contacts. Almost all fungal and oomycete viruses have genomes composed of single-stranded or double-stranded RNA, and recent studies have revealed numerous novel viruses representing yet unclassified family-level groups. Depending on the virus–host combination, infections can be asymptomatic, beneficial or detrimental to the host. Thus, mycovirus infections may contribute to the multiplex interactions of hosts, therefore likely affecting the dynamics of fungal communities required for the functioning of soil ecosystems. However, the effects of fungal and oomycete viruses on soil ecological processes are still mostly unknown. Interestingly, new metagenomics data suggest an extensive level of horizontal virus transfer between plants, fungi and insects.

Phytophthora cactorum is an important oomycetous plant pathogen with numerous host plant species, including garden strawberry (Fragaria × ananassa) and silver birch (Betula pendula). P. cactorum also hosts mycoviruses, but their phenotypic effects on the host oomycete have not been studied earlier. In the present study, we tested polyethylene glycol (PEG)-induced water stress for virus curing and created an isogenic virus-free isolate for testing viral effects in pair with the original isolate. Phytophthora cactorum bunya-like viruses 1 and 2 (PcBV1 & 2) significantly reduced hyphal growth of the P. cactorum host isolate, as well as sporangia production and size. Transcriptomic and proteomic analyses revealed an increase in the production of elicitins due to bunyavirus infection. However, the presence of bunyaviruses did not seem to alter the pathogenicity of P. cactorum. Virus transmission through anastomosis was unsuccessful in vitro.