A Comparison of Five Similar Cytoplasmic Hypovirulent Strains ofEndothia parasiticafrom the Southern Appalachian Mountains

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Abstract
Five culturally similar, debilitated, dsRNA-containing strains, isolated from American chestnut trees growing in the southern Appalachian Mountains and presumed to be Endothia parasitica, were studied to establish if they were E. parasitica and determine and compare the factors that contribute to their hypovirulence (sensu lato), i.e., subnormal pathogenicity, fruiting capacity, or both. These strains were compared with a strain from western Michigan that has similar abnormalities conferred by dsRNA-associated cytoplasmic hypovirulence (CH) agent HM1. In American chestnut the five southern Appalachian strains were nonpathogenic, like strains containing CH agent HM1, or very weakly pathogenic. Each produced only two types of single-conidial isolates: Isolates like the original strain (usually less than 20%) and isolates with characteristics typical of E. parasitica. DsRNA-free single-conidial isolates from all five southern Appalachian strains had cultural characteristics typical of E. parasitica and normal levels of virulence (sensu lato) in American chestnut [Castanea dentata] as determined by comparing them with four standard strains in culture and in American chestnut trees in the field over a 12-mo period. With one minor exception, each of these single-conidial isolates produced perithecia and ascospores typical of E. parasitica in the field and in mating type tests. The five southern Appalachian strains fell into four vegetative compatibility groups. The cytoplasmic agent in each strain was transmitted by hyphal anastomosis directly, or indirectly through intermediary strains, into each of the four standard nuclear genetic backgrounds of E. parasitica. Each standard acquired the cultural abnormalities, hypovirulence, and single-conidial isolate segregation pattern of each original strain. Each agent was associated with a consistent pattern of dsRNA components in the original nuclear genetic background and in the two standard backgrounds studied. Four patterns of dsRNA components were found, and none of these was identical to the pattern associated with CH agent HM1. These findings indicate that each of the five southern Appalachian strains is a strain of E. parasitica with a nuclear genetic background that confers normal cultural characteristics and a normal level of virulence (sensu lato), and a single dsRNA-associated agent confers its abnormalities. The different patterns of dsRNA components suggest that four CH agents are represented among the five southern Appalachian strains and that these agents differ from CH agent HM1 from the Michigan strain. The new agents have been designated HNC1, HT1, HT2, and HV1.