Identifying Hypovirulent Isolates ofCryphonectria parasiticawith Broad Conversion Capacity

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Abstract
When 118 virulent (V) and 27 hypovirulent (H) isolates of C. parasitica were paired in culture, 95% of the V isolates were converted to the hypovirulent condition by at least 1 of the H isolates. The 118 V isolates in 54 vegetative compatibility (v-c) groups included representatives from West Virginia, North Carolina, Virginia, Tennessee [USA] and Italy. The average conversion capacity of the 27 H isolates was 15% and ranged 0-41%. Eight cluster analyses based on different similarity functions were performed to group V isolates according to their susceptibility to conversion. The most useful cluster analysis was based on the square root of the number of sectors converted by individual H isolates and formed 9 conversion groups and left only 8 isolates ungrouped. Twenty-eight of the 54 v-c groups had more than 1 V isolate, and isolates in 15 of these v-c groups were also together in conversion groups. Clustering appears useful for determining relatedness among v-c groups. 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 1 of the H isolate treatments. Conidia were as effective as mycelia in conversion, but more of the colony margin of the V isolates had altered growth from mycelial slurries than from conidial treatments. In a 2nd slurry experiment, minimum numbers of H isolates were selected on the basis of providing maximum conversion of the 118 V isolates in the pairing experiment. Conidial slurries from 4, 7 and 11 H isolates with 82, 91 and 95% conversion of the 118 V isolates in pairings provided conversion of 97, 99 and 85% of the 102 randomly selected, V isolates. Conidial slurries of as few as 4 H isolates with broad conversion capacity have potential for biological control of chestnut blight on American chestnut [Castanea dentata] because they breach the barrier of vegetative incompatibility.