Further Evidence for Two Cytoplasmic Hypovirulence Agents in a Strain ofEndothia parasiticafrom Western Michigan

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Abstract
Correlative evidence supported the hypothesis that two cytoplasmic agents confer the abnormalities of EP-60, the first highly debilitated, dsRNA-containing strain of Endothia parasitica found in North America. The nuclear genetic background of EP-60 conferred normal cultural characteristics and normal virulence (sensu lato) in American chestnut [Castanea dentata], as determined by comparing it with three standard strains. The putative agents were transmitted both separately and together, by hyphal anastomosis and through conidia, in a cycle from the nuclear genetic background of EP-60 to the background of EP-6, an unrelated, marked (methionine-requiring) strain, and back to the nuclear genetic background of EP-60, approximately fulfilling Koch''s postulates for the agents separately and together. Four infection states, the number predicted for two independent cytoplasmic agents, occurred as three cultural types. Each infection state was associated with a different combination of cultural characteristics, pathogenicity, SCI segregation pattern, and pattern of dsRNA components. Each agent conferred consistent cultural abnormalities on 20 other North American and European nuclear genetic backgrounds of E. parasitica, including the three standard backgrounds and seven that previously contained other dsRNA-associated agents. Each agent also similarly affected the pathogenicities and virulences of representative North American and European nuclear genetic backgrounds, including the standards. The infection state of EP-60 was established in the nuclear genetic backgrounds of EP-60, EP-6, and the standards by hyphal anastomosis between corresponding single infected forms. A different, but consistent, pattern of dsRNA components was associated with each agent in the nuclear genetic backgrounds of EP-60 and the three standards. These patterns appeared to be combined when both agents were present. The more and less debilitating agents were transmitted via about 10 and 90% of conidia, respectively. A mycelial slurry containing isolates with both agents had marked curative effects when introduced into naturally occurring cankers. These observations confirm that EP-60 has a nuclear genetic background that is typical of E. parasitica and demonstrate that it contains two independent cytoplasmic agents which, when present alone in many nuclear genetic backgrounds of E. parasitica, confer different degrees of hypovirulence and, when together, have marked curative effects on developing cankers. The more and less debilitating cytoplasmic hypovirulence agents were designated HM1 and HM2, respectively.