Twenty-four-hour Rainfall, a Simple Environmental Variable for Predicting Peanut Leaf Spot Epidemics. D. P. Davis, Postdoctoral Fellow, Department of Plant Pathology, Auburn University, Auburn, AL 36849-5409. J. C. Jacobi, and P. A. Backman. Research Associate, and Professor, Department of Plant Pathology, Auburn University, Auburn, AL 36849-5409. Plant Dis. 77:722-725. Accepted for publication 25 February 1993. 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, 1993. DOI: 10.1094/PD-77-0722.

Data from peanut leaf spot epidemics (caused primarily by Cercosporidium personatum) during the 1983–1986 growing seasons in Alabama were used to develop disease predictive models, and data from 1988 and 1989 were used to validate the models. The percentage of infected leaflets was used as the dependent variable, and independent variables included days after planting (DAP) and days with rainfall exceeding a specified minimum. Three rainfall thresholds were evaluated: 1.27, 2.54, and 6.35 mm/day. The Von Bertalanffy/Richards equation was used to develop all models. The onset of an epidemic was defined as the point when the model predicted infection of 5% of leaflets. Overall, the model based on DAP fit the data well (R² = 0.76), as did the model based on daily precipitation exceeding 2.54 mm (R² = 0.77). The model based on daily precipitation above 6.35 mm was less satisfactory (R² = 0.67), and the use of daily precipitation above 1.27 mm as the explanatory variable did not result in a model fit. Onset of the average epidemic occurred 60 days after planting in the DAP model, or after 6 days with precipitation above 2.54 mm or 5 days with rainfall exceeding 6.35 mm in the precipitation threshold models. The model based on daily precipitation greater than or equal to 2.54 mm performed better in validation tests than the other models. These results provide the rationale for the conclusion that 1-day rainfall of 2.54 mm or more is an easily measured environmental variable that may be used to schedule fungicide sprays for managing peanut leaf spot.