Variation in the pathogenicity of Erysiphe graminis D.C. F. Sp. Avenae,and its relation to the development of mildew-resistant oat cultivars

Two new physiologic races of Erysiphe graminis avenae are described, capable of attacking hitherto resistant hexaploid genotypes of Avena in the seedling stage. Two diploid forms namely A. strigosa var. glabrota and A. strigosa s.sp. hirtula provide the only sources of resistance to the races of mildew isolated in this investigation. In two cultivars, relative degrees of resistance to mildew infection in field trials have been shown to be due to resistance developing in later formed leaves of maturing plants. The importance of developing cultivars with a high degree of adult plant resistance to disease attack in the field is discussed.