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Relative Survival Potential of Propagules of Phythophthora megasperma f. sp. medicaginis. J. P. Stack, former Graduate Research Assistant, and R. L. Millar, Professor, Department of Plant Pathology, Cornell University, Ithaca, NY 14853.  Phytopathology 75:1025-1031.

Pouches of nylon screen containing zoospores, hyphae, sporangia, or oospores of Phythophthora megasperma f. sp. medicaginis were added to an unsterile soil maintained at different levels of temperature (4, 15, and 24C) and moisture (0, –10, –100 mb ψm) to determine the relative survivability of the propagules.  The rates of propagule lysis correlated positively with soil temperature.  There was no statistically significant effect (P = 0.05) of soil moisture upon the rate of lysis.  Complete hyphal lysis occurred in 9 days at 24C and in 15 days at 15C, but took longer than 35 days at 4C.  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 24C, 9 days at 4C, or 16 days at 15C.  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 upon temperature and moisture conditions.  Relative persistence in unsterile field soil of zoospores, hyphae, oospores, or root segments colonized by a metallaxyl-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.