The economical benefit of controlling potato late blight, caused by Phytophthora infestans (Mont.) de Bary, depends on both the quantitative increases in the yield and its effects on quality. The late blight was effectively controlled to combine resistance of varieties, proper cultivation measures and appropriate use of fungicides. The average number of fungicide application in Latvia has been between 2 and 4 times. Mostly, the first field treatment was made during the row closing or according to prognoses of development of potato late blight. Field trials to compare different potato late blight control models were carried out at 5 different sites in Latvia in the years 2001 and 2002. A computer based model was compared with untreated control and routine application of fungicides. Replicated 4-row plots in randomised block design were used in each treatment. The year 2001 was favourable for the development potato late blight. Both variants with fungicide treatments gave a significantly higher yield compare with untreated control. There were not significant differences between the variant of computer based model and the routine. On the other hand, the growing season 2002 was very hot and not favourable for disease development and there were not significant differences between untreated control and variants with fungicide application. The use of computer based model reduced fungicide application and the total amount of
fungicide l ha-1. The outlay of routine fungicide treatments was 37% higher than with computer based models in the growing season 2001 and 58% higher in the growing season 2002.