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Relationship Between Incidence and Severity of Powdery Mildew and Leaf Rust on Winter Wheat. W. C. James, Plant Pathologist, Ottawa Research Station, Canada Department of Agriculture; C. S. Shih, Statistician, Statistical Research Service, Canada Department of Agriculture. Phytopathology 63:183-187. Accepted for publication 18 July 1972. DOI: 10.1094/Phyto-63-183.

Disease incidence and severity data for powdery mildew (Erysiphe graminis DC. ex Mérat f. sp. tritici Em. Marchal) and leaf rust (Puccinia recondita Rob. ex Desm. f. sp. tritici Eriks.) of winter wheat (Triticum aestivum L.) were recorded during three surveys in Ontario in 1969 and 1970. An exponential equation was used to describe the relationship between the incidence (percentage of leaves infected) and severity (percentage of leaf area affected) for the two diseases on particular leaves. A linear regression was found to be adequate to estimate severity for incidence values of 65% or below. The relationship between incidence and severity for each of two diseases was consistent over a large geographical area, but differed for the 2 years.