Progress in Selecting for Resistance to Stemphylium botryosum (Cool‐Temperature Biotype) in Alfalfa1

Abstract

Plants from seven alfalfa, Medicago sativa L., cultivars were inoculated with the cool‐temperature biotype of Stemphylium botryosum Wallr. in a controlled environment chamber to identify plants resistant to Stemphylium leafspot. Seedlings were inoculated at the 6 to 8‐leaf stage, maintained at 18 to 20 C under constant mist for a 48‐hour infection period and scored for disease seven days after inoculation. Resistant plants were intercrossed to form a single population which was subjected to two cycles of phenotypic recurrent selection. Progress after two cycles of selection was evaluated in a greenhouse modified to provide conditions similar to the smaller controlled environment chamber. Relative disease reaction of six test cultivars in the two infection environments was the same (r[4] = 0.978). A proportional bulk population, constructed in proportion to each cultivar's contribution to the first selection cycle, was evaluated as an internal standard to measure progress during selection. Disease resistance increased in each selection cycle compared to the standard proportional bulk population with 10, 59, and 89% resistant plants in the proportional bulk, cycle‐1, and cycle‐2 populations, respectively.