Hypovirulent Isolates ofEndothia parasiticaAssociated with Large American Chestnut Trees

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Abstract
Hypovirulent isolates of E. parasitica were associated with large, chronically infected, American chestnut trees. Normal isolates from such cankers were also commonly obtained. The abnormal isolates varied widely in pathogenicity. Although not all of the hypovirulent isolates had abnormal cultural characteristics, most showed considerable differences in pigmentation, growth rate, mycelial organization, and patterns of segregation of single-conidial isolates compared with normal. Several isolates did not produce perithecia in the field. Segregation of colony types among single-conidial isolates and presence of double-stranded RNA among several isolates suggested that viruslike cytoplasmic agents when responsible for their hypovirulence. The practical value that these isolates might have in the biological control of chestnut blight in still to be determined.