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Gradual Evolution of Virulence of Puccinia coronata on Oats. M. D. Simons, ARS, U.S. Department of Agriculture, Department of Plant Pathology, Iowa State University, Ames 50011; M. E. McDaniel, Department of Soil and Crop Sciences, Texas A & M University, College Station 77843. Phytopathology 73:1203-1205. Accepted for publication 5 April 1983. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1983. DOI: 10.1094/Phyto-73-1203.

Seedlings of Avena sativa ‘TAM 0-301’ in the greenhouse at a mean temperature of 23 C were highly resistant to all isolates of Puccinia coronata from Texas that were tested in 1974. In 1975- 1976, TAM 0-301 was moderately resistant to a few isolates; in 1977- 1979 it was moderately resistant to a greater proportion of isolates and susceptible to some isolates. In 1980, TAM 0-301 was resistant to only 17% of all isolates, moderately resistant to 64%, and susceptible to 19%. In growth chambers, susceptibility and high resistance, as seen in the greenhouse, were not temperature sensitive, but moderate resistance was. Isolates that induced moderately resistant reactions in the greenhouse generally induced highly resistant reactions at 20 C and susceptible reactions at 26 C. The same gene in TAM 0-301 conditioned both high resistance to some isolates and moderate, temperature-sensitive resistance to others.