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Identification of Resistance to Pythium Seedling Diseases in Alfalfa Using a Culture Plate Method. NORA A. ALTIER, former graduate student, Department of Plant Pathology, University of Minnesota, St. Paul55108. JUDY A. THIES, research plant pathologist, ARS, USDA, U.S. Vegetable Laboratory, Charleston, SC 29412. Plant Dis. 79:341-346. Accepted for publication 8 December 1994 . This article is in the public domain and not copyrightable It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1995. DOI: 10.1094/PD-79-0341.

Pathogenicity and virulence of three Pythium spp. to alfalfa seedlings and resistance of alfalfa germ plasm to Pythium seedling diseases were characterized using a culture plate method. Twenty-five seeds were placed on the surface of a 3-day-old colony of Pythium growing on water agar and incubated at 12, 18, or 24 C for 5 days. Disease severity was rated using a 5-class scale (I = healthy seedling, 5 = dead seed). Pathogenic cultures of Pythium ultimum, P. paroecandrum, and a P. sylvaticum-Wke isolate, all from alfalfa fields in Minnesota, varied in virulence to the Beltsville International Composite-7 alfalfa population. Moderately virulent isolates induced greater disease at 12 and 18 C than at 24 C, but highly virulent isolates were uniformly virulent over the three temperatures. Twelve alfalfa entries representing diverse germ plasm backgrounds varied significantly in resistance to P. ultimum and P. paroecandrum. Alfalfa seed weight, seedling growth rate, rate of germination, and fall dormancy were not correlated with this resistance. The culture plate method is useful for evaluating alfalfa germ plasm for resistance to Pythium seedling diseases

Keyword(s): damping-off, seed rot