EXPANSION OF POTATO LATE BLIGHT FORECASTING MODELS FOR THE COLUMBIA BASIN OF WASHINGTON AND OREGON

A regional potato late blight forecasting system for irrigated potatoe s in the semiarid environment of the Columbia Basin was expanded by de veloping specific forecasting models for four vicinities throughout th e Basin. Relationships between weather and outbreaks of late blight at the locations over a 27-year period were examined using logistic regr ession analysis. The response variable was a year either with or witho ut a late blight outbreak. An indicator variable representing the occu rrence of an outbreak during the preceding year (Yp) and number of day s of rain during April and May (Ram) correctly classified the disease status (presence or absence of late blight) of 89, 82, 78, and 78% of the years at Presser, Washington, Hermiston, Oregon, and Hanford and O thello, Washington, respectively. The percentage of years with disease outbreaks correctly classified was 93, 85, 79, and 79% at the four re spective locations. All years with late blight outbreaks and 96% of th e total years were correctly classified using data from at least one o f the four locations. These predictors are particularly important earl y in the growing season and can be used to make area forecasts. A seco nd set of predictors, Yp and number of days of rain in July and August (Rja), for Hermiston and Hanford, and a third set, Yp, Ram, and Rja, for Presser and Othello were found effective for making additional lat e blight forecasts later in the growing season.