PATHOGENIC VARIATION OF RHYNCHOSPORIUM-SECALIS IN DENMARK AND SOURCES OF RESISTANCE IN BARLEY

Thirty-eight Danish isolates of Rhynchosporium secalis, 36 from barley and two from rye, were inoculated onto 23 cultivars of barley, one cultivar of rye, and nine species of noncereal grasses. The isolates were divided into 28 different races (DK1-DK28) according to their reaction with the barley cultivars. These 28 races represent a very broad virulence spectrum. Race 1 caused susceptible reactions in 19 of the 23 cultivars, whereas race 26 caused susceptible reactions in only six cultivars and race 27 in only one cultivar. All barley cultivars were resistant to race 28, represented by the two isolates from rye. Fifteen cultivars of barley with race-specific resistance were included in the present set of differentials. Of these cultivars, only two, Atlas and Osiris, were resistant to all races tested, whereas the rest was susceptible to two or more races. Of eight commercial cultivars, Corgi, Digger, and Hasso were resistant to 27, eight and, eight races, respectively. Four other cultivars were susceptible to all but two of the races: Race 27, virulent on only cultivar Magda and race 28, comprising isolates from rye. The last cultivar was only resistant to race 28. These results suggest that known sources of race-specific resistance to R. secalis may not be effective in Denmark if not used in combination with race-nonspecific resistance. One isolate from barley caused minor infections in rye but not in any of the noncereal grasses. The two isolates from rye caused minor infections in some barley cultivars but the noncereal grasses were not infected. This suggests not only that R. secalis is rather strongly specialized to the host species from which it is isolated, but also that it may infect other species as well.