PATHOGENIC VARIATION OF RHYNCHOSPORIUM-SECALIS IN DENMARK AND SOURCESOF 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 cu ltivar of rye, and nine species of noncereal grasses. The isolates wer e divided into 28 different races (DK1-DK28) according to their reacti on with the barley cultivars. These 28 races represent a very broad vi rulence spectrum. Race 1 caused susceptible reactions in 19 of the 23 cultivars, whereas race 26 caused susceptible reactions in only six cu ltivars and race 27 in only one cultivar. All barley cultivars were re sistant 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 susc eptible to two or more races. Of eight commercial cultivars, Corgi, Di gger, and Hasso were resistant to 27, eight and, eight races, respecti vely. Four other cultivars were susceptible to all but two of the race s: Race 27, virulent on only cultivar Magda and race 28, comprising is olates from rye. The last cultivar was only resistant to race 28. Thes e results suggest that known sources of race-specific resistance to R. secalis may not be effective in Denmark if not used in combination wi th race-nonspecific resistance. One isolate from barley caused minor i nfections in rye but not in any of the noncereal grasses. The two isol ates from rye caused minor infections in some barley cultivars but the noncereal grasses were not infected. This suggests not only that R. s ecalis is rather strongly specialized to the host species from which i t is isolated, but also that it may infect other species as well.