VIEW ARTICLE

Ecology and Epidemiology

Weather and Downy Mildew Epidemics of Hop in Washington State. Dennis A. Johnson, plant pathologist, Department of Plant Pathology, Washington State University, Pullman 99164; J. Richard Alldredge(2), and Jennifer R. Allen(3). (2)(3)associate professor and technical assistant, respectively, Program in Statistics, Washington State University, Pullman 99164. Phytopathology 84:524-527. Accepted for publication 15 February 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/Phyto-84-524.

Relationships between weather and downy mildew epidemics of hop in the Yakima Valley of Washington State over a 38-yr period were examined with linear discriminant and logistic regression analyses. A linear discriminant function with minimum temperature in April × the total number of rainfall days in April and May, an indicator variable for the presence of an epidemic the preceding year, and the minimum temperature in May × the number of occurrences of three or more consecutive rainfall days in May correctly classified the epidemic status for 92% of the years. The percentage of years with mild epidemics correctly classified (specificity) and years with severe epidemics correctly classified (sensitivity) for a 10-yr test data set were 86 and 100%, respectively. Logistic regression, which requires fewer assumptions, had results very similar to those of the discriminant analysis.