1.8.61 INTERSPECIFIC TRANSMISSION OF A HYPOVIRULENCE-ASSOCIATED DOUBLE-STRANDED RNA FROM SCLEROTINIA SCLEROTIORUM TO S. MINORBY HYPHAL ANASTOMOSIS INTERSPECIFIC TRANSMISSION OF A HYPOVIRULENCE ASSOCIATED DOUBLE-STRANDED RNA FROM SCLEROTINIA SCLEROTIORUM TO S. MINOR BY HYPHAL ANASTOMOSIS MS MELZER, SS IKEDA and GJ BOLAND Environmental Biology, University of Guelph,Guelph,Ontario,Canada N1G2W1 Background and objectives Transmissible hypoviruience has been reported in Sclerotinia sclerotiorum (isolate Ss275) where hypovirulence and atypical colony morphology were associated with a double-stranded RNA (dsRNA) [1]. The hypovirulent phenotype was transmissible between vegetatively-compatible isolates of the species, and recipient isolates developed the hypovirulent phenotype. Hypovirulence has been associated with dsRNA in other pathogenic fungi, particularly Cryphonectria parasitica, where at least three hypovirulence-associated dsRNAs have been classified in the new virus family, Hypoviridae. DSRNA from isolate Ss275 of Sclerotinia sclerotiorum has not been critically characterized but may also belong to this family of viruses. Hypovirulence in C. parasitica has contributed significantly to the control of chestnut blight in Europe and the potential for using hypovirulent isolates of other fungal plant pathogens for biological control of selected diseases has been suggested. However, hypovirulence-associated dsRNAs have not been identified in many pathogens, and the efficacy of individual dsRNAs varies. Fungal viruses and dsRNAs are not transmitted extracellularly and, therefore, transmission between isolates occurs only during hyphal anastomosis. Therefore, promising dsRNAs for biocontrol within one pathogen have limited application to other pathogens. Chen et al. [2] successfully infected C. parasitica and several closely related fungal species with synthetic transcripts of hypovirus CHV1-EP713 from C. parasitica but, to our knowledge, there are no reports of transmission of dsRNA between fungal species through hyphal anastomosis. The objective of this study was to attempt interspecific transmission of hypovirulence-associated dsRNA by hyphal anastomosis between hypovirulent isolate Ss275 of S. sclerotiorum [1] and virulent isolates of S. minor. Results and conclusions Transmission of the hypovirulent phenotype and dsRNA from isolate Ss275 of S. sclerotiorum to five isolates of S. minor was attempted. Although many S. minor colonies developed atypical colony morphologies when paired with Ss275, the majority of these colonies regained normal morphology after subculture. However, in 9 of 39 aftempts, isolate Sml0 of retained a typical morphology. Three of nine isolates of Sml0 that were putatively converted to hypovirulence were selected for evaluation in a virulence assay on detached lettuce leaves. Isolates Sml0(G), Sml0(I), Sml0(T) and Ss275 all produced significantly smaller lesions than the original isolate Sml0, Sml0(G), Sml0(I) and Sml0(T) were evaluated for the presence of dsRNA and all three isolates contained a band of dsRNA consistent in size with the dsRNA in isolate Ss275. This dsRNA was not detected in the original isolate Sml 0. In northern hybridizations, dsRNA from isolate Sml0(T) hybridized with a CDNA probe prepared from dsRNA of Ss275. RAPD analyses of genomic DNA confirmed that isolate Sml O(T) was derived from isolate Sml 0 and was not derived from Ss275 or from a hybrid of the two species. The dsRNA detected in Sml0(T) was subsequently transmitted to isolate Sm8, an isolate of S.minor in the same vegetative compatibility group as Sml0. lntraspecific transmission between these isolates was confirmed by northern hybridization. This is the first report of interspecific transmission of a hypovirulence-associated dsRNA in fungi by hyphal anastomosis. The results confirm that interspecific transmission of dsRNA and the hypovirulent phenotype can occur among populations of Sclerotinia species in vitro. The potential for interspecific transmission among populations of Sclerotinia species in vivo, and the influence of interspecific transmission on the population ecology of dsRNA and hypovirulence in these fungi, remains to be determined. References 1. Boland GJ,1994. Can. J. Plant Pathol. 14,10-17. 2. Chen C, Chein-Hwa C, Bowman BH, Nuss DL, 1996. Phytopathology 86, 301-310.