Survival of Phytophthora cinnamomi in forest soils in bottles, as measured by successful reisolation, dropped steadily during the first 6-months following inoculation, and remained nearly constant at a very low level up to 19 months, after which it could not be isolated. Frequency of recovery was greatest in agricultural soil, but survival was not longer than in forest soil. The presence of small Douglas-fir (Pseudotsuga menziesii) seedlings did not prolong survival time. Chlamydorores are regarded as the principal survival structures. Mycelial growth was inhibited throughout the year in both forest and cultivated soil. The fungus, a poor competitive soil saprophyte, failed to colonize Douglas-fir twigs when the surrounding soil mixture contained less than 50 percent alfalfa mealsand inoculum. Contrarily, in sterilized forest soil growth was rapid at moisture levels of 43 and 58 percent and at temperatures between 15 and 25°C. Zoospores germinated with subsequent sporangial formation on fir root tips at 15 and 30°C. In unsterilized soil filtrate, zoospores swam to Douglas-fir roots, 3 inches away, but in wet soil, less than 1 inch was transversed. Prolonged low soil moisture was fatal to the fungus.
Document Type: Journal Article
Formerly Assistant in Plant Pathology, Oregon State Univ., is now Plant Pathologist, Southeastern Forest Expt. Station. Tech.