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 w ith linear discriminant and logistic regression analyses. A linear dis criminant function with minimum temperature in April X the total numbe r of rainfall days in April and May, an indicator variable for the pre sence of an epidemic the preceding year, and the minimum temperature i n May X the number of occurrences of three or more consecutive rainfal l days in May correctly classified the epidemic status for 92% of the years. The percentage of years with mild epidemics correctly classifie d (specificity) and years with severe epidemics correctly classified ( sensitivity) for a 10-yr test data set were 86 and 10 %, respectively. Logistic regression, which requires fewer assumptions, had results ve ry similar to those of the discriminant analysis.