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Evaluation of Potato Late Blight Forecasts Modified to Incorporate Host Resistance and Fungicide Weathering. W. E. Fry, Department of Plant Pathology, Cornell University, Ithaca, NY 14853; A. E. Apple(2), and J. A. Bruhn(3). (2)(3)Department of Plant Pathology, Cornell University, Ithaca, NY 14853, Present address of third author: Biochemical Department, E. I. du Pont de Nemours & Co., Wilmington, DE 19898. Phytopathology 73:1054-1059. Accepted for publication 11 January 1983. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-1054.

Host resistance was incorporated into two potato late blight forecasts, Blitecast and a computer simulation model-generated forecast. Each forecasting system recommended that less frequent fungicide sprays be applied to resistant than to susceptible cultivars. Modifications of Blitecast were based on field estimates of fungicide weathering and cultivar resistance and were conservative; adjustments in fungicide dosage were insufficient to compensate for differences between moderately resistant and susceptible cultivars. The simulation forecast was derived from analysis of two simulation models. One model described weather effects on fungicide distribution and amount. The second model described weather and host resistance effects on development of the pathogen, Phytophthora infestans. The simulation forecast accurately incorporated host resistance and fungicide effects late blight was suppressed similarly on susceptible and resistant cultivars. During the course of these experiments, differences in host resistance caused greater differences in disease development than did variation in weather.