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

Abstract

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 selfs 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 transplanted 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.