Comparative rainfastness assessment of different copper-based fungicides

Copper antimicrobial properties have been well known since the times of ancient Egyptians, who used it to sterilize water and wounds. In the modern era, after the fungicidal properties of the Bordeaux mixture were discovered by Millardet in 1885, several fungicides were developed using different copper compounds as active substances. The long-term success of these products is linked to their high efficacy at affordable costs and the low risk of developing resistance in the target plant pathogens. However, they can be used only as contact fungicides, and most of the active substances can be washed out from the crop canopy by rainfall. Modern agrochemistry allowed the development of new formulations with increasing rainfastness. Therefore, the old assumption that copper fungicides are not helpful in the context of frequent and heavy rainfalls is no longer valid. This study assessed the rainfastness of seven products containing different copper compounds and formulation technologies after applying a simulated rain of 80 mm on grapevine potted plants. Fungicides implementing the newest formulation technology indicated an increase of metallic copper available on the leaf surface of 18.3-76.7% to the standard Bordeaux mixture and 77.5-158.5% to copper sulphate pentahydrated. Parallel efficacy trials done on Plasmopara viticola using the same products revealed a higher efficacy than the reference in heavy rainfalls, consuming less metallic copper per hectare. These results demonstrated that new copper-based fungicides withstand the challenges of extreme weather events and are essential tools for organic farming and IPM strategies.