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Vegetative Compatibility and Hypovirulence Conversion Among Naturally Occurring Isolates of Cryphonectria parasitica. E. G. Kuhlman, Research Plant Pathologist, USDA Forest Service, Southeastern Forest Experiment Station, Carlton Street, Athens, GA 30602; H. Bhattacharyya, Mathematical Statistician, USDA Forest Service, Southeastern Forest Experiment Station, Carlton Street, Athens, GA 30602. Phytopathology 74:659-664. 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-74-659.

Virulent (V) isolates of Cryphonectria parasitica were readily recovered from both sunken cankers and swollen superficial cankers on American chestnut trees in the southern Appalachians. Hypovirulent (H) isolates of C. parasitica made up 38% of the population in six swollen superficial cankers on one isolated tree in Tennessee but were infrequently recovered from 9 sunken and 52 swollen superficial cankers from North Carolina, Virginia, and Italy. A cluster analysis was used to group V isolates by vegetative compatibility (v-c) and by conversion susceptibility. Isolates from the Tennessee tree had four distinct v-c groups identified by the merge-barrage response and each canker averaged 2.7 v-c groups. Forty-one cankers on 19 trees near Buchanan, VA, yielded 2.3 v-c groups per canker. Pairings among 93 randomly selected isolates from these within canker groups indicated 17 v-c groups were present in the 303-m-long study area, with an average range of 134 m per v-c group. Susceptibility to conversion by six H isolates from the area was present in 80% of the 93 isolates. Although susceptibility to conversion was widespread (average 217 m per H isolate), the H isolates occurred in only four of 41 cankers over a distance of 56.7 m. V isolates were identified with broad susceptibility to conversion by H isolates from several v-c groups and H isolates were identified with broad capacity for conversion of V isolates from several v-c groups. When American chestnut saplings were inoculated with H isolates, 52% were live and healed over, 28% were live and infected, and 20% had dieback to the inoculation point in 27- 29 mo. Inoculation of saplings with V isolates resulted in 3% live and 97% with dieback.

Additional keywords: Castanea dentata, chestnut blight, Endothia parasitica, Koch' s postulates.