Evolution of resistance to Rhynchosporium secalis (Oud.) Davis in barley composite cross II.

Changes in resistance to scald disease which occurred in barley composite Cross II over 45 generations were analyzed genetically. This population, which was synthesized in 1929 by pooling equal numbers of f1 seeds from 378 pair wise crosses among 28 barley varieties, has subsequently been grown at Davis, California under standard agricultural conditions without conscious selection. Progenies derived from self-pollinated seeds from random plants taken from four generations (F8, F13, F23, and F45) were tested against four different races of scald (40, 61, 72, and 74), and rated as resistant, susceptible or segregating. Striking increases in the frequency of families resistant to races 40, 61, and 74 occurred in CC II. A test for randomness showed that quadruply susceptible and triply resistant families were more common than expected under the assumption that resistance to different races is independent. Positive correlations were found between resistance to races 40, 61, and 74, but resistance to race 72 was independent of resistance to all other races. Possible reasons for these correlations are discussed.