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Pathogenic Variation in Rhynchosporium secalis from Southern Ontario. G. Xue, Research Associate, and R. Hall, Professor, Department of Environmental Biology, and D. Falk, Assistant Professor, Department of Crop Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1. Plant Dis. 75:934-938. Accepted for publication 26 February 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0934.

Among 352 isolates of Rhynchosporium secalis collected from barley (Hordeum vulgare) in Ontario during 1987, 1988, and 1989, 20 races were identified on the basis of virulence to five barley differentials, cvs. La Mesita, Turk, Trebi, Abyssinian, and OAC Elmira. The races were labeled SO1 to SO20 in order of increasing number of differentials matched. SO1, the most common race, composed 55% of the population and was virulent to the barley breeding line GW8614 and two Ontario winter barley cultivars, OAC Acton and OAC Halton, but to none of the differentials. The remaining 19 races were virulent on one to five differentials. None of the races was virulent to cv. Atlas or Atlas-46. Of the cultivars grown commercially in Ontario, OAC Acton and OAC Halton were susceptible to all races and OAC Elmira was resistant to nine races. The frequency of races was inversely related to the number of differential cultivars matched.