in Moscow, ID. He received a B.S. degree in forest management (1963) and Ph.D degree in plant pathology (1969) from Washington State University. Since joining the Station in 1966, he has studied the epidemiology and genetic interaction of the blister rust organism and its hosts. He is now studying the ecological genetics of conlfer-Armillaria interactions.
Over all plots (disturbed and pristine), incidence of pathogenic Armillaria showed a strong tendency to decrease as habitat type productivity increased. This trend gave rise to a clear separation of plots by climax series. The relatively less productive subalpine fir and Douglas-fir series exhibited high incidence of root disease and the relatively more productive grand fir, western redcedar, and western hemlock series significantly less. Within these productivity groups, other patterns emerged. Disturbance appeared to be related to a dramatic increase in incidence of pathogenicity, but not occurrence, within the high-productivity grouping of communities. Also, the ability of disturbance to elicit pathogenicity seemed to decline as site productivity increased. Conversely, the pristine plots within the lowproductivity series exhibited high incidence of the pathogen in a pathogenic state. This condition seemed to be related to a community structure characteristic of transition between cold-dry to cool-moist and warm-dry to warmmoist. Predicting risk of Armillaria-cawsed mortality, occurrence of pathogenic species and clones of Armillaria, a possible role for host stress in expression of pathogenicity by Armillaria, and risk rating of host species are discussed. KEYWORDS: habitat types, host stress, root rot management, root rot risk, risk rating, disease hazard Armillaria commonly occurs as rootlike rhizomorphs growing on plant debris or epiphytically attached to root systems of dead, diseased, or healthy host plants (Garrett 1960; Kile 1980; Leach 1939; Raabe and Trujillo 1963; Redfern 1973). Isolates obtained from such rhizomorphs, as well as isolates obtained from mycelial fans, decayed wood, and sporophores, can belong to clones or species whose apparent pathogenicity varies from very high to obligately saprophytic (Kile 1983; Rishbeth 1982; Wargo and Shaw 1985).
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