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Geographical Distribution and Associations Between Resistance to Four Races of Rhynchosporium secalis. Qifa Zhang, Post-doctoral researcher, Department of Genetics, University of California, Davis 95616; R. K. Webster(2), and R. W. Allard(3). (2)Professor, Department of Plant Pathology, University of California, Davis 95616; (3)Professor, Department of Genetics, University of California, Davis 95616. Phytopathology 77:352-357. Accepted for publication 23 July 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-352.

Data from an assay of a random sample of 350 accessions of barley from the USDA world collection for reaction to four physiological races (races 40, 61, 72, and 74) of Rhynchosporium secalis were analyzed using a disease index, Shannon’s information statistic, and product-moment correlation. Accessions from different geographical areas differ in degree of resistance to the four races individually and in levels of diversity for resistance to the four races jointly. The races studied differ in aggressiveness, which was negatively associated with racial complexity. Race 40 (the most simple race) was the most aggressive race and race 74 (the most complex race) was the least aggressive. The proportions of the total variability accounted for by various components of the hierarchical geographical structure are: differences among different regions 4%, differences between countries within regions 6%, differences between accessions within countries 37%, and heterogeneity within accessions 53%; the factors responsible for the large within-accession diversity are unknown. Resistance to races 40, 61, and 74 was positively associated, but resistance to race 72 was independent of the other three races.