In the Midwestern United States, potato late blight (Phytophthora infestans) is a temporally sporadic disease, occurring only when microclimate conditions within the canopy are favorable and inoculum is present. Favorable environmental conditions include air temperatures between 7 and 27 degrees C. and relatively long periods (10 hours or more) of leaf wetness. Increasing concern in the agricultural community over observed and projected climate change has prompted numerous studies on the possible implications for crop yields, however, relatively little work has focused on disease management. In the Upper Great Lakes region, historical climatological trends point to warmer and wetter growing season conditions, with significant trends in precipitation totals and in the frequency of days with precipitation. These types of change could increase late blight risk and subsequent crop failure. However, it is not possible to evaluate the effect of change without thoroughly understanding the historical trends in of late blight risk.

Historical trends in hourly weather variables and potato late blight risk as expressed by the Wallin Disease Severity Value index were analyzed at 7 stations in Wisconsin, Michigan, and Ohio from 1948-1999. All sites showed significant trends in at least one of the risk estimates. While late blight risk is greatest at all stations in August, periods of increasing risk have occurred across the region, particularly during July. The increases in disease risk appear to be associated with upward trends in dry bulb and dew point temperature at nearly all of the stations, particularly during July and August.