Weather and downy mildew epidemics of hop in Washington state.

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 X 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 X 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 [...]