Vegetative Compatibility and Hypovirulence Conversion Among Naturally Occurring Isolates ofCryphonectria parasitica

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Abstract
Virulent (V) isolates of C. parasitica were readily recovered from both sunken cankers and swollen superficial cankers on American chestnut Casta trees in the southern Appalachians. Hypovirulent (H) isolates of C. parasitica made up 38% of the population in 6 swollen superficial cankers on 1 isolated tree in Tennessee [USA] but were infrequently recovered from 9 sucken and 52 swollen superficial cankers from North Carolina, Virginia [USA] 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 4 distinct v-c groups identified by the merge-barrage response and each canker averaged 2.7 v-c groups. Cankers (41) on 19 trees near Buchanan, Virginia, yielded 2.3 v-c groups/cankers. 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 6 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 4 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 isolated 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.

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