Improvement of late blight management in organic potato production systems in Europe: field tests with more resistant potato varieties and copper based fungicides

Late blight of potatoes, caused by Phytophthora infestans, is particularly difficult to prevent or control in organic agriculture. In this study, the host resistance of selected varieties to foliar and tuber blight and their yield under organic growing conditions were assessed in trials carried out in four countries (Switzerland, France, England and Norway) in 2001 and 2002. The objective was to identify new, more resistant varieties which might replace some of the more susceptible varieties which are currently grown in organic agriculture. In each country, five test varieties were compared with two reference varieties, identical for all sites, and with two locally popular varieties. In addition, the effect of copper fungicides was assessed for all varieties in England. Several new potato varieties (Appell, Derby, Innovator and Naturella in Switzerland; Derby, Eden, Escort and Naturella in France; Sarpo Axona, Eve Balfour, Lady Balfour, Sarpo Mira and Sarpo Tominia in England; N89-1756 and N92-15138 in Norway) proved far less susceptible to foliar and tuber blight than most of the varieties currently grown. However, market requirements could restrict their introduction into practice. The copper fungicide treatment reduced foliar blight severity in all varieties tested in England and in both years, by 27% on average, and increased yield by 20% on average, but did not affect tuber blight. In organic farming, a shift towards the use of blight resistant varieties should thus be strongly encouraged, although this is unlikely to eliminate the need to use copper fungicides