Threshold Relative Humidity Duration Forecasts for Plant Disease Prediction

Duration of high relative humidity periods is an important component of many plant disease development models. Performance of forecasts of this quantity, based on the model output statistics 3-h temperature and dewpoint forecasts produced by the National Meteorological Center, is examined for selected locations. A probability model for high relative humidity duration conditional on these forecasts is presented, which succinctly describes forecast performance and would be straightforward to implement operationally. The forecast humidity duration intervals are relatively broad, but their transformation into probability forecasts of disease severity ratings, illustrated for the case of potato late blight, can produce good results.