Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 28, 1996
Publication Date: N/A
Interpretive Summary: Alfalfa is a valuable crop that farmers can grow to produce high-quality hay or forage to feed to livestock. However, alfalfa is not widely grown in the southeastern USA partly because it is often seriously damaged by a fungus disease that is common in this region. This disease, called Sclerotinia crown and stem rot, is caused by a fungus that attacks plants in the winter. Infected leaves and stems are rotted away, and plants may be killed or greatly weakened by early spring. In years when this disease is severe, yields of hay and forage available to the farmer are greatly reduced. The economic losses caused by Sclerotinia crown and stem rot could be prevented if scientists could breed new varieties of alfalfa that resist infection by the fungus. To do this, they must accurately identify the most resistant plants and cross them. Seed produced by the parent plants are then retested to try to find more resistant progeny, and the cycle is repeated until highly resistant plants are obtained. Scientists have not been able to do these steps effectively because no reliable methods were available to identify small numbers of resistant plants in large populations. In this study, new techniques were developed to test individual leaves for resistance to the fungus. When low numbers of resistant plants were identified by leaf testing and crossed, their progeny were shown to be much more resistant than the original plants and other progeny not selected for resistance. This study shows that leaf tests can be used by plant breeders to develop resistant populations of alfalfa. These techniques may provide the necessary steps to enable scientists to develop new varieties of alfalfa for the southeastern USA that will not be destroyed by the Sclerotinia disease.
Technical Abstract: Excised leaf tissues of alfalfa were inoculated with Sclerotinia trifoliorum to determine whether leaf inoculation techniques may be used to screen for heritable and effective resistance to the pathogen. Leaf tissues were incubated on agar in Petri plates and inoculated with mycelium of S. trifoliorum. Host responses were evaluated according to the rate and extent of necrotization of leaf tissue that occurred during 1 wk at 17 C. One thousand plants of cultivar Delta were screened for resistance by progressive inoculation of excised unifoliate leaves, first trifoliate leaflets, and discs of tissue from later leaves. The five plants considered most resistant were polycrossed, and progeny were compared with those of five plants selected for susceptibility, five plants selected at random, and the parent cultivar. Progeny of plants selected for resistance manifested significantly (P=0.05) less severe disease than did progeny of plants selected for susceptibility, plants selected at random, and the parent cultivar, when evaluated by inoculation of unifoliate leaves, first trifoliate leaflets, leaf discs, excised stems, and whole plants. With natural infection in the field, progeny of resistant plants developed less severe disease and produced significantly (P=0.05) higher yields of forage through the first or second harvests than did progeny of susceptible plants, random plants, and the parent cultivar in three of four experiments conducted during two growing seasons. These results establish that inoculation of excised leaf tissue is an effective and efficient means to screen for resistance to S. trifoliorum in alfalfa.