Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/31/1995
Publication Date: N/A
Interpretive Summary: Sclerotinia crown and stem rot is the name of a serious disease of crops of alfalfa and clovers that are grown to produce hay or forage. This disease is caused by a soil fungus that invades the leaves and stems during the winter and causes tops of plants to die by early spring. Infected plants may be killed by the fungus and yields of hay or forage greatly reduced. These economic losses to farmers would be prevented if scientists can breed new varieties of alfalfa and clovers that resist the disease or only become slightly damaged by it. In this study, a population of alfalfa with a high level of resistance to the Sclerotinia crown and stem rot disease was developed for the first time. To create this population, large numbers of plants were infected with the fungus that causes the disease in the laboratory, and a few that did not develop strong disease symptoms were cross-pollinated to produce seed. Plants grown from this seed were again infected, and the least diseased plants were again cross-pollinated. The seed from this second generation of selected plants were infected with the disease and found to be more resistant than all of 24 varieties of alfalfa that are recommended to be grown in the southeastern United States. Therefore, seed from the second generation of plants selected for resistance are now being released to plant breeders for their use in developing new varieties of alfalfa that could have more resistance to Sclerotinia crown and stem rot.
Technical Abstract: MSR (Mississippi Sclerotinia-resistant) alfalfa (Medicago sativa L.) germplasm was jointly released by ARS and the Mississippi Agricultural and Forestry Experiment Station in 1995. MSR is the first germplasm of alfalfa developed for resistance to Sclerotinia trifoliorum Eriks., the causal agent of Sclerotinia crown and stem rot of alfalfa and other forage legumes. MSR was produced as a by-product of efforts to develop techniques to screen alfalfa for resistance to S. trifoliorum. Initially 1,675 plants of cultivar Delta were screened by stem- and leaf-inoculation techniques. Twenty-five plants were selected and polycrossed, and progeny were evaluated for resistance. Nine of the most resistant progeny, out of 2,772 screened, were selected and polycrossed to produce MSR. In artificial inoculation experiments, MSR was significantly more resistant (P<0.01) than all of 24 entries included in the 1992-93, 1993-94, and 1994-95 Southern Regional Alfalfa Variety Tests. MSR also expressed resistance in comparison to Delta when inoculated with isolates of S. trifoliorum from Louisiana, Pennsylvania, Ohio, and Wisconsin, and with S. sclerotiorum (Lib.) de Bary from Georgia and Wisconsin. In field experiments at Mississippi State, where all disease resulted from natural infection, severity of disease in individual plants of MSR was less than in Apollo, Cimarron, Delta, Florida-77, and WL-320 when disease pressure was sufficient. In a yield comparison of fungicide-treated and nontreated plots, MSR attained 83% of its yield potential in nontreated plots where disease development was unimpeded, whereas Apollo, Delta, and Florida-77 attained 38-49% of their yield potentials.