VIEW ARTICLE

Ecology and Epidemiology

Incidence and Severity Relationships of Secondary Infections of Powdery Mildew on Apple. R. C. Seem, Department of Plant Pathology, New York State Agricultural Experiment Station, Cornell University, Geneva 14456; J. D. Gilpatrick, Department of Plant Pathology, New York State Agricultural Experiment Station, Cornell University, Geneva 14456. Phytopathology 70:851-854. Accepted for publication 1 February 1980. Copyright The American Phytopathological Society. DOI: 10.1094/Phyto-70-851.

Severity (amount of leaf tissue affected by disease) can be determined from incidence (proportion of leaves diseased) for secondary infections of apple powdery mildew (caused by Podosphaera leucotricha) on apple (Malus pumila) leaves. A simple relationship was derived in which √severity was proportional to incidence. The equation was developed iteratively although a similar solution was arrived at by making the variance of incidence and severity data independent of their respective means. The relationship remained constant for data from different locations or cultivars, but it varied according to different seasons. The relationship remained relatively constant for leaves of different age although younger leaves developed the most disease. Mildew severity can be derived by measuring incidence on the five to 10 youngest terminal leaves. For quantitative severity assessment, adjustment must be made for differing apple cultivars and/or growing seasons.