Evaluation of the curative and protectant activity of fungicides and fungicide–adjuvant mixtures on Phomopsis cane and leaf spot of grape: A controlled environment study

Phomopsis cane and leaf spot of grape—an economically important disease of grapes in many regions of the world—can be controlled in Ohio by applying fungicide mixed with adjuvant in response to infection periods predicted with the use of a disease-warning system. A series of experiments was conducted in this study to determine the possible curative activity of fungicides against Phomopsis viticola when used with certain adjuvants. Grape plants were sprayed with fungicide (with or without adjuvant) either before or after inoculation with the pathogen. Leaf and internode disease severity for all post-inoculation treatments were not different from that found for the inoculated (unsprayed) control, even when fungicides were used at 150% of labeled doses, or when fungicides were applied within 20 h of the start of the infection period or 4 h from the end of the wetness period. However, excellent disease control was obtained with pre-inoculation application of mancozeb or azoxystrobin, with experiments that varied in duration of the wetness periods for infection. Use of an adjuvant did not improve disease control. Thus, suppression of disease symptoms in the field based on the use of the disease-warning system was likely due to the purely protective activity of the fungicide.