Changes in the distribution of phenylamide-resistant strains ofPhytophthora infestans (Mont.) de Bary in Ireland 1985–89

Summary Phenylamide-based fungicides were reintroduced to the Irish market in 1985. Mixtures of phenylamides and mancozeb were recommended for no more than 3 sprays in potato late blight control programmes with the last systemic spray to be applied not later than 31 July. Samples of potato foliage infected withP. infestans were collected from the main potato growing areas in Ireland over a five-year period starting in 1985. Using the floating disc method, 11% of the crops sampled in 1985 were found to have phenylamide-resistant strains ofP. infestans present. The number of crops with resistant strains increased rapidly up to 1988 when 83% of the crops sampled had resistance. Factors affecting the distribution of resistance and the efficacy of a phenylamide-based spray programme are discussed.