Recurrent Selection for Bacterial Wilt Resistance in Alfalfa1

Abstract

A‐C4 and B‐C4, two unrelated populations of alfalfa, Medicago sativa L., were subjected to recurring cycles of phenotypic selection for resistance to bacterial wilt caused by Corynebacterium insidiosum (McCull) Jens. Thirty‐five hundred to 5,000 seedlings were inoculated in each population per cycle. About 150 plants in each population were intercrossed to initiate each new cycle of selection. The disease severity indices for wilt infection (0 = healthy, 5 = dead) for A‐C4 and after one and two cycles of selection were 3.72, 2.53, and 1.38, respectively. Comparable indices for B‐C4 and after each of four cycles of selection were 4.25, 3.94, 3.67, 3.13, and 2.63. Wilt indices for Narragansett,’ ‘Ranger,’ and ‘Vernal’ were 4.46, 3.04, and 2.17, respectively. Recurrent phenotypic selection was highly effective for developing resistance to bacterial wilt. Different genetic mechanisms for resistance in the two populations were postulated.