Characterization of a mycovirus associated with the brown discoloration of edible mushroom, Flammulina velutipes

Background: A mycovirus previously identified in brown discolored fruiting bodies of the cultivated mushroom Flammulina velutipes was characterized. We tentatively named the virus the F. velutipes browning virus (FvBV). Results: Purified FvBV particles contained two dsRNA genomes (dsRNA1 and 2). The complete sequence of dsRNA1 was 1,915 bp long, containing a single open reading frame (ORF) that encoded 580 amino acids of a putative 66-kDa RNA-dependent RNA polymerase (RdRp). dsRNA2 was 1,730 bp long containing a single ORF encoding 541 amino acids of a putative 60-kDa coat protein (CP1). Phylogenetic analysis of the RdRp sequences revealed FvBV to be a Partitivirus, most closely related to Chondrostereum purpureum cryptic virus. An RT-PCR assay was developed for the amplification of a 495-bp cDNA fragment from dsRNA encoding the CP1. When wild F. velutipes isolated from various parts of Japan were examined by RT-PCR assay, three isolates from the central region of Japan contained FvBV. One wild strain infected with FvBV was isolated in Nagano prefecture, where brown discoloration of white cultivated strains has occurred. Fruiting bodies produced by virus-harboring and virus-free F. velutipes were compared. Conclusions: Cap color of the fruiting bodies of F. velutipes that contained Partitivirus FvBV was darker than FvBV-free fruiting bodies. The use of RT-PCR enabled association of FvBV and dark brown color of the fruiting body produced by F. velutipes strains.