Hypovirulent isolates of E. parasitica [flight pathogen of Castanea sativa and C. dentata] were cured of detectable levels of double-stranded (ds) RNA by treatment with cycloheximide. Curing was accompanied by a dramatic increase in virulence, and by recovery of a normal colony morphology. Two isolates were resistant to cycloheximide-induced curing, but when their dsRNA were transferred to other nuclear genetic backgrounds, curing was possible. All cured strains could be converted to the hypovirulent form by conversion with a hypovirulent strain. Converted strains gained dsRNA and displayed decreased virulence.