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The Dynamics of a Controlled Population of Rhynchosporium secalis, Changes in Race Composition and Frequencies. L. F. Jackson, Postgraduate Research Plant Pathologist, Department of Plant Pathology, University of California, Davis 95616; R. K. Webster, Professor, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 66:726-728. Accepted for publication 23 December 1975. DOI: 10.1094/Phyto-66-726.

The dynamic nature of the California population of Rhynchosporium secalis was revealed when a mixture of five isolates representing five races of widely varying pathogenicity was carried through two successive disease cycles and an intervening saprophytic stage in the greenhouse on the susceptible barley cultivar Numar. One hundred single-spore isolates obtained after the second disease cycle were characterized on 14 barley differentials and only 17 of them were found to belong to the original five races. The 83 remaining isolates represented 14 hybrid-type races. Most of these isolates differed from one or more of the original isolates in pathogenicity on only one or two of the differentials, although some of them differed on up to six of the differentials. In addition to the generation of new races, there was a shift in the fungus population toward the more simple pathogenic races and away from the more complex pathogenic races. The original isolates differed in sporulating ability in culture and on host tissue, but differences were not related to the respective pathogenic complexities of the isolates.

Additional keywords: barley, disease resistance, pathogenic variability, scald of barley.