Effect of two double-stranded RNA viruses on the virulence of the phytopathogenic fungus Fusarium oxysporum.

Fusarium root rot, a persistent soil-borne disease, poses a serious threat to crop production, quality, and ultimately to food security. We identified two double-stranded RNA viruses co-infecting the phytopathogenic fungus Fusarium oxysporum strain 3 S-18: Fusarium oxysporum partitivirus 1 isolate 3 S-18 (FoPV1/3S18) and Fusarium oxysporum virus 1 (FoV1). The genome of FoPV1/3S18 consists of two segments. dsRNA1 is 1,761 nt in length with a large open reading frame (ORF) encoding an RNA-dependent RNA polymerase (RdRp) of 539 amino acids (aa). dsRNA2 is 1,556 nt in length with an ORF encoding a putative coat protein (CP) of 430 aa. Phylogenetic analysis based on both RdRp and CP amino sequences indicated that FoPV1/3S18 clusters with the members of the genus Gammapartitivirus within the family Partitiviridae. FoV1 was identified as a new monopartite dsRNA virus with 2,944 nt, containing two ORFs which encode a protein of 590 aa RdRp and 134 aa nucleocapsid protein, respectively. It belongs to the genus Unirnavirus, family Amalgaviridae. Furthermore, we demonstrated that both FoPV1/3S18 and FoV1 can be successfully transmitted via hyphal anastomosis to a virus-free strain. Co-infection with FoV1 and FoPV1/3S18 reduced conidial production but did not attenuate fungal virulence. However, infection with FoV1 alone not only reduced conidial production but also induced hypovirulence.