The nucleotide sequences of two double-stranded (ds) RNAs (L- and S-dsRNAs) in the hypovirulent isolate Sh12B of 'Sclerotinia homoeocarpa', the causal agent of dollar spot of turfgrass, were determined. The S-dsRNA was 732-738 base pairs (bp) in length and sequence analysis indicated that it was a satellite dsRNA. The L-dsRNA was 2,632 by in length and one strand contained an open reading frame (ORF) with the potential to encode a protein of 720 amino acids. This ORF contains conserved motifs of RNA-dependent RNA polymerases (RdRp). This L-dsRNA was homologous with mitovirus dsRNAs and shared 92.4% nucleotide and 95.1% amino acid sequence identities with the 'Ophiostoma novo-ulmi mitovirus' 3a-Ld (OnuMV3a) from ' Ophiostoma novo-ulmi', the causal agent of Dutch elm disease. These results suggest that the dsRNAs are conspecific, and the L-dsRNA was designated ' O. novo-ulmi mitovirus' 3a-Sh12B (OnuMV3a-Sh12B). This is the first report that a hypovirulence-associated dsRNA virus occurs naturally in two taxonomically distinct fungi, and indicates that horizontal transmission of this virus may have occurred between these two fungi, despite their separate geographic locations in Europe (OnuMV3a-Ld) and Canada (OnuMV3a-Sh12B). In ' S. homoeocarpa', attenuation of virulence during storage was associated with latent infection by OnuMV3a and 29.8% of the latently-infected isolates were converted to a hypovirulence phenotype during 21 months of storage at 4°C. Transmission of OnuMV3a from latently-infected to virus-free isolates resulted in latent infection of the recipient isolates, indicating that latent infection by OnuMV3a was not associated with genetic differences in the fungal host. Sequence comparison of the OnuMV3a viruses in an isogenic pair of latently infected and hypovirulent isolates revealed high sequence identities (95.8% nucleotide and 94.9% amino acid). Vegetative incompatibility in populations of 'S. homoeocarpa' was investigated, and three types of incompatibility reactions and four vegetative compatibility groups (VCGs) were characterized among 116 isolates originating from eastern Canada. Transmission of OnuMV3a revealed that some types of incompatibility reactions allowed limited transmission, indicating that vegetative incompatibility was not an absolute barrier to the transmission of OnuMV3a. The results from the present study support the use of hypovirulence as a biocontrol strategy in dollar spot management.