CHARACTERISTICS OF HYPOVIRULENT STRAINS OF CHESTNUT BLIGHT FUNGUS, CRYPHONECTRIA PARASITICA ISOLATED IN KOREA

Chestnut blight disease caused by Cryphonectria parasitica is widely distributed throughout chestnut tree plantations in Korea. We surveyed 65 sites located at 9 provinces in south Korea, and isolated 248 virulent and 3 hypovirulent strains of chestnut blight fungus. Hypovirulent strains had dsRNA virus in the cytoplasm, which is one of the typical characteristics of hypovirulent strains. In addition, they showed more characteristics of hypovirulent strains, i.e., suppressed conidiation, reduced pigmentation in colony color, and reduced phenol oxidase activity as well as reduced pathogenicity. Hypovirulent strains, KCPH-22 and KCPH-136, had only one dsRNA band with the molecular weight of 2 kb, but KCPH-135 had one more dsRNA band with the molecular weight of 0.7 kb. Single condia were isolated from the cultures of hypovirulent strains to investigate the viral heredity in asexual reproduction. Various phenotypes and absence of dsRNA bands were obtained from single conidial cultures, which means that hypovirulence transmission is unstable in asexual reproduction. Biocontrol trials in the field using hypovirulent strains were also carried out.