Potato late blight: A new approch for an outdoor screening of substitutes for copper fungicides

Microbial antagonists and plant or compost extracts have a potential as environmental friendly substitutes for copper to control late blight in potatoes in organic farming. Altough plant compounds have been shown to be able to inhibit Phytophthora infestans in vitro and in greenhouse trials, even the best of them failed to control potato late blight under field conditions. Reasons for failures are probably insufficient rain fastness, UV-stability, and/or inappropriate application due to insufficient knowledge of the properties of the agents. We developed a semi-field screening system to fill the gap between greenhouse and the field and to determine the effectiveness of plant compounds under varying climatic conditions. Extracts of the three Chinese medicinal herbs Terminalia chebula, Anemarrhena asphodeloides, Galla chinensis as well as copper (Kocide DF) were sprayed on potato plants in the field. Leaves were detached at different time points and infected in the laboratory. The protective effect of T. chebula and A. asphodeloides was similar to a treatment with a low copper concentration (100 g Cu per ha). G. chinensis did not control the disease at all. An exposition of more than 48 hours under field conditions as well as rainfall strongly reduced the effectiveness of the plant extracts. In an ordinary field trial T. chebula inhibited the disease just as well as the low copper concentration. Formulations and application strategies have to be adapted to improve the effectiveness of the tested products. The semi-field screening system is a promising approach to speed up the testing procedure and the development of new antifungal agents based on plant compounds.