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 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 (1 = healthy seedling, 5 = dead seed). Pathogenic cultures of Pythium ultimum, P. paroecandrum, and a P. sylvaticum-like 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.