CHV3-type dsRNAs and the GH2 genotype in a population ofCryphonectria parasiticain Ontario

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Abstract
CCP19, a hypovirulent isolate of Cryphonectria parasitica collected from a site near Bothwell, Ontario, in 1987, has shown potential for use in biological control of chestnut blight. CCP19 contains dsRNA that hybridizes with hypovirus CHV3 from isolate GH2 (CHV3-GH2), a hypovirulent isolate of C. parasitica from Michigan. In 1995, 26 isolates of C. parastica were collected at the Bothwell site, and 22-23 (85-89%) of these 26 isolates had significantly smaller lesion diameters in an apple virulence assay than virulent control isolate EP155. Twenty-two of the 26 isolates (85%) contained dsRNA. Segments of dsRNA in all 22 isolates hybridized with cDNA prepared from CHV3-type dsRNA in CCP19. The CHV3-type dsRNA was present in all three MCGs identified in the population. DNA fingerprint analysis revealed a highly clonal population, Twenty isolates had DNA fingerprint genotypes the same as, or similar to, GH2 and 6 isolates had genotypes similar to CCP19. Similarities, including presence of CHV3-type dsRNA, highly clonal populations, and low numbers of MCGs, were noted between this site and sites where hypovirulence is prevalent in Michigan. Two isolates with the hypovirulent phenotype, CCP47 and CCP52, have potential for biological control of chestnut blight.