Historical weather data of recent seven years from 22 weather stations in Switzeriandwere analysed according to crucial weather conditions for the epidemic of potato late blight(Phytophthora infestans). Close relationship was found between disease incidence and the crucial weather conditions. Checking the growing season with crucial weather conditions by treatment intervals of 6, 9 and 12 days, the treatment times for each potato growing region wereachieved. June has been proved to be the most important month for both disease developmentand chemical treatment. Although because of the great variation of the weather, such suggestions are rather general. It is beneficial for people to understand the ecological difference amongdifferent regions and to get a perfect disease management.