Characteristics of dsRNA-Free and dsRNA-Containing Strains ofEndothia parasiticain Relation to Hypovirulence

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Abstract
DsRNA-free strains of E. parasitica and dsRNA-containing strains derived from abnormal chestnut blight cankers in France, Italy and North America were compared to determine if simple reliable cultural indicators of dsRNA and low pathogenicity (hypovirulence [sensu stricto]) could be identified. The prototrophic dsRNA-free strains produced nearly identical colonies in culture, large lesions in apples, and large cankers with abundant stromata and spore tendrils in excised dormant Amercian chestnut. Those tested in the field produced large cankers with abundant stromata and perithecia. In contrast, the dsRNA-containing strains all differed in culture from the dsRNA-free strains and from one another, all but one were deficient in pathogenicity and fruiting capacity, and their deficiencies spanned a wide range. Few produced perithecia. These observations suggest: simple, reliable cultural indicators of dsRNA and low pathogenicity probably do not exist, but overall appearance in culture may be a useful criterion for selecting strains to be tested for dsRNA; field tests spanning more than 6 mo. are needed to detect the deficiencies of some dsRNA-containing strains, dsRNA is not invariably associated with subnormal pathogenicity and fruiting capacity in E. parasitica; a variety of factors may contribute to the diversity found among the dsRNA-containing strains and the natural recovery phenomenon; and natural concepts and definitions of virulence and hypovirulence in E. parasitica are needed that accommodate this diversity. Broad concepts and operational definitions of virulence and hypovirulence (sensu lato), based on the pathogenicities and fruiting capacities of typical dsRNA-free strains (standards), are offered and discussed, and an approach is proposed for determining the factors which contribute to the hypovirulence (sensu lato) of individual strains.