IDENTIFICATION OF RESISTANCE TO PYTHIUM SEEDLING DISEASES IN ALFALFA USING A CULTURE PLATE METHOD

Pathogenicity and virulence of three Pythium spp. to alfalfa seedlings and resistance of alfalfa germ plasm to Pythium seedling diseases wer e characterized using a culture plate method. Twenty-five seeds were p laced 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 wa s rated using a 5-class scale (1 = healthy seedling, 5 = dead seed). P athogenic cultures of Pythium ultimum, P. paroecandrum, and a P. sylva ticum-like isolate, all from alfalfa fields in Minnesota, varied in vi rulence 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 ove r the three temperatures. Twelve alfalfa entries representing diverse germ plasm backgrounds varied significantly in resistance to P. ultimu m and P. paroecandrum. Alfalfa seed weight, seedling growth rate, rate of germination, and fall dormancy were not correlated with this resis tance. The culture plate method is useful for evaluating alfalfa germ plasm for resistance to Pythium seedling diseases.