Inhibition of oomycetes by the mixture of maleic acid and copper sulfate
After the discovery of the protective activity of Bordeaux mixture against plant disease caused by oomycetes, copper compounds have been used for over a century as a significant plant protection strategy. However, application of excessive copper can cause adverse effects through long-term heavy metal accumulation in soils. Therefore, it is necessary to develop new strategies to reduce or replace copper in pesticides based on organic and low-input farming systems. Organic acids are eco-friendly in nature. In this study, we tested the antifungal and anti-oomycete activity of maleic acid (MA) and copper sulfate (CS) against thirteen plant pathogens. Treatment of MA and CS mixture showed strong anti-oomycetes activity against Phytophthora cambivora, P. capsici and P. cinamomi. Moreover, the concentration of CS in the activated mixture of MA and CS was lower than that in the activated CS only, and the mixture showed synergy or partial synergy effects on the anti-oomycete activity. Application of a wettable powder formulation of MA and CS mixture (MCS 30WP; 26.67 % MA and 3.33 % CS) exhibited excellent protective activities in pot experiments with control values of 73 % Phytophthora blight on red pepper, 91 % damping-off on cucumber, and 84 % Pythium blight on creeping bentgrass, which are similar to those of the CS wettable powder formulation (6.67 % CS) containing two times the CS content in MCS 30WP. These observations suggest that the synergistic effect of the MA and CS combination is a sustainable alternative for effective management the destructive oomycete diseases.