Identifying hypovirulent isolates of Cryphonectria parasitica with broad conversion capacity.

When 118 virulent (V) and 27 hypovirulent (H) isolates of Cryphonectria parasitica were paired in culture, 95% of the V isolates were converted to the hypovirulent condition by at least one of the H isolates. The 118 V isolates in 54 vegetative compatibility (v-c) groups included representatives from West Virginia, North Carolina, Virginia, Tennessee, and Italy. The average conversion capacity of 27 H isolates was 15% and ranged from 0 to 41%. Conidial or mycelial slurries of 7, 15, and 27 H isolates converted 87-93% of 102 randomly selected V isolates from North Carolina. All 102 V isolates were converted by at least one of the H isolate treatments. Conidia were as effective as mycelia in conversion. Conidial slurries of as few as four H isolates with broad conversion capacity have potential for biological control of chestnut blight on American chestnut because they breach the barrier of vegetative incompatibility.