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Snow Rot of Winter Wheat in Washington. P. E. Lipps, Research Assistant, Department of Plant Pathology, Washington State University, Pullman, WA 99164; G. W. Bruehl, professor, Department of Plant Pathology, Washington State University, Pullman, WA 99164. Phytopathology 68:1120-1127. Accepted for publication 27 February 1978. Copyright 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-1120.

Pythium aristosporum, P. iwayamai, P. ultimum, and an unidentified Pythium sp. (Pythium sp. d) were isolated from dead wheat plants after snow and ice melt in eastern Washington. Leaves and crowns were rotted, and leaf sheaths at the base of tillers were browned. Plants with snow rot occurred in the same fields as plants affected by snow molds, but they were confined to areas where water had collected during snow and ice melt. Spraque winter wheat (which is moderately resistant to snow mold) was susceptible to snow rot, indicating that resistance to snow rot and to snow mold is not correlated. The Pythium spp. were isolated from plant tissues incubated at 1 C. At 0.5 C, P. iwayamai was pathogenic to Nugaines winter wheat when wet absorbent cotton was used to simulate snow on saturated and drained rooting medium. Pythium aristosporum was pathogenic with or without the cotton cover under saturated conditions but only moderately pathogenic under drained conditions. Pythium ultimum was nonpathogenic to plants under snow rot conditions, but it invaded root and leaf tissues without inciting snow rot. Pythium sp. d was isolated most frequently from plants associated with ice, but pathogenicity tests were not attempted. Pythium iwayamai killed 50% of the plants after 56 days under snow rot conditions at 0.5 C, but failed to incite root rot in the greenhouse at 8-15 C. Pythium aristosporum killed 50% of the plants after 72 days under snow rot conditions, and also caused severe root rot at 8-15 C. Pythium iwayamai and P. aristosporum grew more rapidly at 0.5 C than did Typhula idahoensis or Fusarium nivale. On agar media adjusted to varying osmotic potentials, P. aristosporum grew most rapidly at 1.3 bars (the highest water potential tested). Pythium ultimum and P. iwayamai grew most rapidly at 3 to 6 bars. Growth of the Pythium spp. was restricted at 28 bars. Pythium iwayamai produced zoospores at 0-15 C, but none at 20 C. Although P. aristosporum produced few zoospores at 5-15 C, it produced none at 0 or 20 C; direct germination of sporangia occurred at 0-20 C. Pythium iwayamai (and probably Pythium sp. d) are considered true snow rot pathogens; P. aristosporum and P. ultimum are not.