EN
Genetic changes in population of Phytophthora infestans influence the time of potato late blight appearance in potato crop. The late blight monitoring system makes it possible to evaluate the reliability of forecasts provided by decision support systems that are used in potato protection and in consequences to protect potato crops in practice against earlier appearing infections. In the paper, results of a monitoring of early and late blight early infections in potato fields in Poland, in the years 2008–2010 was shown. The first symptoms of the both diseases appeared at the end of May and beginning of June, the earliest in the west-south region.