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Comparative Pathogenicity of Isolates of Sclerotinia trifoliorum and S. sclerotiorum on Alfalfa Cultivars. Robert G. Pratt, USDA, ARS, Forage Research Unit, P.O. Box 5367, Mississippi State, MS 39762. Dennis E. Rowe, USDA, ARS, Forage Research Unit, P.O. Box 5367, Mississippi State, MS 39762. Plant Dis. 79:474-477. Accepted for publication I February 1995. 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-79-0474.

Eight cultivars and one experimental population of alfalfa were artificially inoculated with five isolates each of Sclerotinia trifoliorum and S. sclerotiorum. Isolates of both species originated from different forage legume hosts and geographic areas in the United States. Inoculations were performed by dusting dried and comminuted mixtures of infested wheat and oat grain over foliage of 4-wk-old plants. Plants were maintained at 17-20 C with intermittent atmos-pheric saturation for 24 days after which plant survival was evaluated. Isolates of both Sclerotinia species differed significantly (P < 0.01) in virulence. Alfalfa cultivars differed significantly (P = 0.02) in susceptibility, and responses of cultivars to the two species were generally similar. Florida 77 was the most susceptible of the eight cultivars to both Sclerotinia species, and 5472 was the least susceptible. An experimental population (STR), previously selected from cultivar Delta for resistance to S. trifoliorum, expressed the least susceptibility to both Sclerotinia spp. Cultivar x isolate interactions were not significant for S. sclerotiorum but were significant (P < 0.01) for S. trifoliorum. These interactions appeared to be caused by differences in virulence of isolates and did not suggest the occurrence of pathogenic races. Significant (P < 0.01) experiment ? cultivar and experiment ? isolate interactions also occurred for both species; possible causes are discussed. Results indicate that responses of these alfalfa cultivars to S. trifoliorum and S. sclerotiorum are generally similar, that selection for resistance to S. trifoliorum in alfalfa may also confer resistance to S. sclerotiorum, that no evidence for different pathogenic races was detected among the isolates, and that host of origin is not an important determinant for the virulence of Sclerotinia isolates on alfalfa.