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Identifying Hypovirulent Isolates of Cryphonectria parasitica with Broad Conversion Capacity. E. G. Kuhlman, Principal plant pathologist, USDA Forest Service, Southeastern Forest Experiment Station, Athens, GA 30602; H. Bhattacharyya(2), B. L. Nash(3), M. L. Double(4), and W. L. MacDonald(5). (2)Mathematical statistician, USDA Forest Service, Southeastern Forest Experiment Station, Athens, GA 30602; (3)Graduate student, School of Forestry, Duke University, Durham, NC 27706; (4)(5)Research associate and professor, Department of Plant Pathology and Agricultural Microbiology, West Virginia University, Morgantown, WV 26506. Phytopathology 74:676-682. Accepted for publication 13 January 1984. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1984. DOI: 10.1094/Phyto-74-676.

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 the 27 H isolates was 15% and ranged from 0 to 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 nine conversion groups and left only eight isolates ungrouped. Twenty-eight of the 54 v-c groups had more than one 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 one 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 second 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 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.

Additional keywords: Castanea dentata, Endothia parasitica.