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ARS Home » Research » Publications at this Location » Publication #52175


item PRATT ROBERT G - 6406-15-10
item Rowe, Dennis

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/1995
Publication Date: N/A
Citation: N/A

Interpretive Summary: Alfalfa is a valuable crop that farmers can grow to produce high-quality hay or forage to feed to cattle and horses. Despite its value, alfalfa is not grown widely in the southern United States because it can be seriously damaged or destroyed by two closely related fungus diseases that are common in this region. The fungi that cause these diseases are present in soil, and they infect the leaves of alfalfa and cause plants to rot and die in winter and spring. These diseases are termed "Sclerotinia" diseases after the scientific name of the fungi that cause them. Recently scientists have had some success in developing varieties of alfalfa that are partly resistant to one of the Sclerotinia diseases. Plants of these new resistant varieties are attacked by the Sclerotinia fungus that causes the disease, but they are not badly damaged, and relatively few of them are killed in comparison to other alfalfa varieties. However, in the southern USA, both of the two Sclerotinia diseases may attack and kill alfalfa in a similar manner, and it was not known whether varieties that are developed for resistance to one disease will also be resistant to the other. In this study, plants of nine alfalfa varieties were infected with different strains of the fungi that cause both Sclerotinia diseases. The number of plants of each variety that were killed by each fungus were determined. The varieties that were the most resistant, with the fewest plants killed, and the most susceptible, with the most plants killed, were largely the same for both diseases. Therefore, these results indicate that if scientists develop alfalfa with resistance to one of the Sclerotinia diseases, it will probably be resistant to the other as well.

Technical Abstract: Eight cultivars and one experimental population of alfalfa were artifically 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 atmospheric saturation for 24 days when 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 Pioneer 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 x cultivar and experiment x 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, and that no evidence for different pathogenic races was detected among the isolates.