Evaluation of Old Potato Late Blight Forecasting Rules During 1994-1999 in Fields with the New Phytophthora infestans Population in Norway

Experiments were conducted to evaluate potato late blight forecasting models in Norway during 1994-1999. Various adjustments of the Førsund rules (FØR) and the NEGFRY model were compared with routine fungicide applications. In the small plot experiments ( n =25) only the fungicide fluazinam was used, but in the large plot experiments ( n =13) several fungicides were included according to farmers' practice. A total of seven cultivars were included in the experiments, Beate and Saturna being the principal ones. Generally, fungicide treatments applied in accordance with the straightforward FØR provided acceptable control of leaf and tuber blight. Using forecast techniques, the number of treatments was reduced by 25-50% compared to routine treatments without causing significantly more disease. There were no significant differences in late blight control and treatment frequency between forecasts based on FØR, using historical weather data, and NEGFRY, using historical weather data and weather prognoses. The implications of the new pathogen population, cultivar resistance and farmers' acceptance of fungicide application being related to use of forecasts are discussed.