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Disease Control and Pest Management

Fungicide Influence on the Relationship between Incidence and Severity 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(2), and R. C. Pearson(3). (2)(3)Department of Plant Pathology, New York State Agricultural Experiment Station, Cornell University, Geneva 14456. Phytopathology 71:947-950. Accepted for publication 2 January 1981. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-947.

The previously quantified relationship between disease severity (amount of leaf tissue affected by disease) and disease incidence (proportion of diseased leaves) for powdery mildew on apple was evaluated under different fungicide control regimes. Data were selected to eliminate external effects due to seasonal, cultivar, and location variation. Five fungicide treatments including captan, benzimidazoles, pyrazophos plus captan, bupirimate with or without captan, and triadimefon produced statistically equivalent incidence/severity (I/S) relationships. The I/S relationship for benzimidazole plus oil was significantly different from that for bupirimate, with or without captan, and for triadimefon. The I/S on untreated trees was significantly different from all fungicide relationships except benzimidazole plus oil and pyrazophos plus captan. Mode of fungicidal action may affect the I/S relationship. A re-analysis of fungicide efficacy data demonstrated how the general relationship could be used to evaluate different treatments based on severity ratings derived from incidence data.