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Inheritance of Resistance to Bacterial Wilt in Two Alfalfa Gene Pools: Qualitative Analysis¹

Bacterial wilt caused by Corynebacterium insidiosum (McCull.) H. L. Jens. is a serious disease of alfalfa (Medicago sativa L.). Inheritance studies were conducted with germplasm from two diverse gene pools, MSA and MSB. Two sets of six-parent diallels with reciprocals and sells were produced within each gene pool. Two sets of parents from one gene pool were crossed with two sets of parents from the other. Eleven-week-old seedlings of progenies were inoculated with C. insidiosum and trans. planted to the field. Fifteen weeks later the taproot of each plant was cross-sectioned and scored for the amount of yellow-brown discoloration.

MSA contained one dominant gene, BW₁, for resistance. In the absence of other genes for resistance, plants with the genotype BW₁... were classified as resistant and bw₁bw₁bw₁bw₁ plants were classified as susceptible. In addition to the BW₁ gene, we believe that MSA contained two other genes, BW₂ and BW₃ with additive and smaller gene effects than those from BW₁. The BW₃ gene had a larger effect than the BW₂ gene. The MSB gene pool did not contain resistant alleles at the BW₁ locus, but it did contain resistant alleles at the BW₂ and BW₃ loci. Based on the genetic origins of the two gene pools, we suggest that the BW₁ gene originated from ‘Turkistan’ and/or ‘Ladak.’ The BW₂ and BW₃ genes were probably from Chilean origin.

Key Words: Medicago sativa L. • Corynebacterium insidiosum (McCull.) H. L. Jens. • Disease resistance • Tetrasomic inheritance

² Formerly graduate research assistant, Dep. of Agronomy and Plant Genetics, Univ. of Minnesota (now assistant professor, Dep. of Plant Breeding, Cornell Univ., Ithaca, NY 14850) and research geneticist, USDA, SEA, AR, Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, St. Paul, MN 55108.

Received for publication November 30, 1978.