This experiment was carried out on a potato (Solanum tuberosum) crop to evaluate a prediction system to control late blight caused by Phytophthora infestans. The trial was conducted from May to September 1998 using the cultivar Bintje. Dimethomorph, cymoxanil, mancozeb and chorothalonil were associated with different spraying schemes, based on the modified Wallin (1962) prediction system and on the traditional late blight control system. The prediction schemes tested consisted of accumulated disease severity values (DSV) of 12, 14, and 16. The traditional late blight control consisted of weekly applications of fungicides starting two weeks after plant emergence, while those defined by the prediction system were carried out based on the DSV accumulated throughout the experiment. Descriptive and diagrammatic scales were used to assess late blight severity. The efficiency of the spray schemes was compared with the area below the curve of disease progress and tuber yield. The treatments based on the prediction scheme were as efficient as the weekly calendar program in reducing disease intensity. Disease severity values of 12 and 14 allowed reduction in the number of sprays without significantly affecting the control level of the disease and tuber yield. The use of the systemic fungicide alone, based on the prediction scheme, resulted in high disease intensity; the use of protective fungicides at weekly intervals gave intermediate control similar to the weekly spray calendar. The prediction schemes based on DSV of 14 reduced dimethomorph sprays without any yield loss.
