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Inheritance of Resistance to Bacterial Wilt in Two Alfalfa Gene Pools: Response to Selection and Quantitative Analysis¹

Bacterial wilt (BW) caused by Corynebacterium insidiosum (McCull.) H. L. Jens. is a serious disease of alfalfa (Medicago sativa L.). High levels of resistance to BW in alfalfa are available, but the inheritance of resistance is unclear. Objectives were to study the response to selection for resistance to BW and conduct a quantitative genetic analysis of inheritance of resistance to BW in two unrelated alfalfa gene pools. In the study of response to selection, the original gene pool plus six cycles of selection in the MSA gene pool and the original plus eight cycles of selection in the MSB gene pool were evaluated simultaneously. In the genetic study three sets of six-parent diallels, including reciprocals and selfs, were produced within each gene pool. Two sets of six-parent Design II's were produced by crossing parents of one gene pool with parents of the other.

The results from the selection study suggested different modes of inheritance in the two gene pools. The response to selection in the MSA gene pool was quadratic, with a rapid increase in resistance in early cycles of selection; the response in MSB was linear and less rapid. In the genetic study variation due to reciprocals was nonsignificant in both gene pools and specific combining ability was significant only in MSB. Assuming a fixed model, all variances of genetic effects were greater in MSB than in MSA. General combining ability effects also suggested that inheritance was different in the two gene pools. The different types of inheritance suggested why the two gene pools responded differently in the selection study.

Key Words: Medicago sativa L. • Corynebacterium insidiosum (McCull.) H. L. Jens. • Diallel • Design II • Resurrent phenotype selection

² Graduate student (now assistant professor, Dep. of Plant Breeding, Cornell Univ., Ithaca, NY 14850) and professor, Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, St. Paul, MN 55108.

³ Research geneticist AR, SEA, USDA in the Dep. of Agronomy and Plant Genetics and research plant pathologist AR, SEA, USDA in the Dep. of Plant Pathology, respectively, Univ. of Minnesota.

Received for publication November 30, 1978.