Adaptation of Phytophthora infestans to partial resistance in potato:: Evidence from French and Moroccan populations

The use of partially resistant cultivars should become an essential component of a sustainable management strategy of potato late blight, caused by Phytophthora infestans. It is therefore important to determine to what extent P.infestans populations can be selected for increased aggressiveness by potato cultivars with different levels of partial resistance. To this end, we sampled P infestans populations from France and Morocco, chosen as locations where late blight occurs regularly but which differ in the distribution of potato cultivars. Cross-inoculation experiments were used to determine the aggressiveness of all populations to potato cvs. Bintje (prevalent in France but not grown in Morocco) and Desiree (popular in Morocco but cultivated to a very small extent in France). French populations were more aggressive on cv. Bintje than on cv. Desiree, irrespective of the site they were sampled from. Their aggressiveness increased between early and late samplings, suggesting that both cultivars selected for increased aggressiveness during epidemics. By contrast, Moroccan populations were more aggressive on Desiree, regarded as partially resistant in Europe, than on Bintje, highly susceptible under European conditions. These data indicate that P. infestans populations adapt to locally dominant cultivars, irrespective of their resistance levels, and can therefore overcome polygenic, partial resistance. This adaptive pattern may render partial resistance nondurable if not properly managed.