Studies on the resistance of the outer cortical tissues of the tubers of some potato cultivars to Phytophthora infestans

Abstract

The outer layers of the cortex of the tubers of some potato cultivars are very resistant to colonization by Phytophthora infestans. Inoculation of this tissue, after its exposure by the removal of the surface periderm, leads to the development of arrested thread-like lesions. The factors responsible for the resistance appear to be restricted to a thin layer of cortical tissue, approximately 30 cells deep, because resistance is removed in the thickness of tissue normally removed from the surface of a tuber by a domestic potato peeler. The lesions produced are similar in appearance to the thread-like necrotic lesions which form in whole tubers of certain cultivars, from infections penetrating through the eyes, lenticels, or wounds which breach the periderm. Although the development of the necrosis is similar to that of a hypersensitive reaction, except that many more cells are involved in the development of each lesion, unlike hypersensitive resistance it is not inhibited by inhibitors of respiratory metabolism such as diethyldithiocarbamate. Furthermore, diethyldithiocarbamate, at a concentration of 10 m m, inhibited the browning reactions associated with the development of the lesion without breaking resistance. Thus outer cortical resistance appears to be based on passive, not active mechanisms of resistance. However the resistance is not a permanent feature of the tissue for considerable reduction in its level of activity occurred in most resistant cultivars during storage.