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Ecology and Epidemiology

Infectivity of Pythium spp. Zoospores in Snow Rot of Wheat. P. E. Lipps, Former research assistant, Department of Plant Pathology, Washington State University, Pullman 99164, Present address of senior author: Ohio Agricultural Research and Development Center, Wooster 44691; G. W. Bruehl, professor, Department of Plant Pathology, Washington State University, Pullman 99164. Phytopathology 70:723-726. Accepted for publication 18 January 1980. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-723.

Pythium iwayamai and P. okanoganense were isolated from naturally infected wheat plants only after water from snow melt had drained through the field. Infections by zoospores of P. iwayamai resulted in the death of wheat seedlings if leaves were inoculated and plants were maintained under flooded conditions at 0.5 C in the dark for a total of 90 days. The length of time plants were maintained at 0.5 C under water after inoculation influenced disease development more than did either the length of time plants remained under these conditions prior to inoculation or the amount of inoculum used (500, 1,000, 5,000, or 10,000 zoospores per milliliter). Zoospores of both P. iwayamai and P. okanoganense encysted on guard cells of stomata; germ tubes growing from the cysts penetrated the stomatal apertures. More zoospores encysted around stomata of old leaves than of young leaves. In the field, zoospores of P. iwayamai and P. okanoganense are released into the snow melt water and zoospores accumulate on plant surfaces and penetrate via stomata.