Variation in tuber pathogenicity of Phytophthora infestans in the Netherlands

Variation in aggressiveness to tubers among isolates of Phytophthora infest ans sampled from three potato growing regions in the Netherlands was compar ed. Variation in the ability to infect tubers of cv. Bintje was found betwe en isolates of each of the three regional populations. The most aggressive isolate of the old population matched the average level of the new populati on in its ability to infect tubers. As a consequence, the commonly used ref erence isolate VK 6C can no longer be considered to be representative of th e present population of P. infestans. Therefore it is recommended that test ing tuber resistance for the official list of potato cultivars with this is olate should be discontinued. Tuber infection and subsequent spread of the fungus in the tuber tissues we re not found to be correlated. The components of tuber pathogenicity studie d were not correlated to pathogenicity factors in the foliage, as measured under growth chamber conditions.