VIEW ARTICLE

Ecology and Epidemiology

Relative Survival Potential of Propagules of Phytophthora megasperma f. sp. medicaginis. J. P. Stack, Former graduate research assistant, Department of Plant Pathology, Cornell University, Ithaca, NY 14853; R. L. Millar, professor, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Phytopathology 75:1398-1404. Accepted for publication 3 May 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-1398.

Pouches of nylon screen containing zoospores, hyphae, sporangia, or oospores of Phytophthora megasperma f. sp. medicaginis were added to an unsterile soil maintained at different levels of temperature (4, 15, and 24 C) and moisture (0, - 10, - 100 mb φ_m) to determine the relative survivability of the propagules. The rates of propagule lysis were positively correlated to soil temperature. There was no statistically significant effect (P = 0.05) of soil moisture on rate of lysis. Complete hyphal lysis occurred in 9 days at 24 C and 15 days at 15 C but took longer than 35 days at 4 C. Encysted zoospores, hyphae developed from germinated cysts, and sporangia responded similarly to hyphae. Oospores survived at least 140 days in soil. Oospores were capable of germination in unsterile soil after 6 days at 24 C, 9 days at 4 C, or 16 days at 15 C. Hyphal lysis and formation of new hyphae occurred concurrently in soil at all three temperatures; some of these new hyphae gave rise to sporangia and/or oospores, depending on temperature and moisture conditions. Relative persistence in unsterile field soil of zoospores, hyphae, oospores, or root segments colonized by a metalaxyl-insensitive isolate (Pm20) of P. m. f. sp. medicaginis was determined by means of two alfalfa seedling bait bioassays. Only oospores free in soil or in colonized root tissue appear capable of long-term survival.