Infectivity ofPythiumspp. Zoospores in Snow Rot of Wheat

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Abstract
P. 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 [Triticum aestivum L. ''Nugaines''] if leaves were inoculated and plants were maintained under flooded conditions at 0.5.degree. C in the dark for 90 days. The length of time plants were maintained at 0.5.degree. 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, 1000, 5000, or 10,000 zoospores/ml). Zoospores of 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.