Assessment of hypovirulent isolates of Cryphonectria parasitica for potential in biological control of chestnut blight

Hypovirulent isolates of Cryphonectria parasitica were evaluated for virulence, conversion capacity, percent transmission of hypovirulent phenotype to conidia, and hybridization relationships among double-stranded RNA (dsRNA) associated with hypovirulent isolates. Significant differences in lesion diameters were detected among hypovirulent isolates in comparison to known virulent and hypovirulent isolates when tested on apple fruit (cv. Golden Delicious) and excised pieces of chestnut bark. Results from the virulence tests on apple fruit and on excised chestnut bark pieces were highly correlated (r2 = 0.80-0.85). Thirteen mycelial compatibility groups (MCGs) were detected among 35 isolates of C. parasitica from southern Ontario, and hypovirulent isolates from Ontario were able to convert two of these groups to the hypovirulent phenotype in vitro. Selected hypovirulent isolates from other locations converted an additional six MCGs. Dramatic differences in percent transmission of the hypovirulent phenotype to conidia were detected among hypovirulent isolates. Transmission to conidia among hypovirulent isolates from Ontario grown on PDA and on excised chestnut sprout pieces ranged from 0 to 96% and 0 to 90%, respectively. Two isolates produced variable results. Transmission among hypovirulent isolates from other locations ranged from 0 to 100%. Random-primed complementary DNA (cDNA) probes constructed from dsRNA extracted from selected hypovirulent isolates hybridized with selected hypovirulent isolates from Ontario and Michigan. The results confirm that hypovirulent isolates of C. parasitica are present in Ontario populations of the pathogen; that significant differences exist among hypovirulent isolates in virulence, conversion capacity, and percent transmission of the hypovirulent phenotype to conidia; and that at least one hypovirulence-associated dsRNA from Ontario is similar to dsRNA from hypovirulent isolate GH2 from Michigan.