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Pythium spp. are known to survive in soil for a long time without a colonizable substrate or host plants or when environmental conditions are not suitable for active growth2). Such ability is dependent on the formation of oospores and other resistant propagules in the soil3,11). Although ecological traits differ markedly among Pythium species8,9), the behavior of propagules under non-planted field conditions still remains unknown. For the ecological study of Pythium propagules in field soil, we recently developed a soil pasteurization method that uses a minimum level of steam to limit native populations of Pythium spp. in soil without any applicable change in other soil micro flora. A replenishment experiment with known levels of Pythium was used for quantitative measurements15,16). Previous observations of oospore germination and hyphal lysis of P. aphanidermatum (Edson) Fitzp. and P. spinosum Sawada in this pasteurized soil indicated that the fungal propagules behaved the same way in this artificially infested soil as in naturally infested soil17,18). This paper compares propagule behavior of P. aphanidermatum and P. spinosum introduced into this pasteurized field soil under non-planted field conditions and demonstrates that propagule density of P. spinosum fluctuated in closer association with changes in natural weather conditions than did that of P. aphanidermatum. A preliminary experiment was previously reported6). Hyphal tips were cultured from one isolate each of P. aphanidermatum (UOP390) and P. spinosum (UOP391), obtained by the baiting method using spinach seedlings from a vegetable field on the University Farm, Osaka Prefecture University (Sakai) in 199219). Their patho-

Propagule Survival of Pythium aphanidermatum and P. spinosum under Non-planted Field Conditions.