Na. Altier et Ja. Thies, IDENTIFICATION OF RESISTANCE TO PYTHIUM SEEDLING DISEASES IN ALFALFA USING A CULTURE PLATE METHOD, Plant disease, 79(4), 1995, pp. 341-346
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.