Influence of formulations and adjuvants on the rainfastness of maneb and mancozeb on pea and potato

The rainfastness of the residual fungicides maneb and mancozeb was assessed on pot‐grown pea (Pisum sativum L) and potato (Solanum tuberosum) plants using a rain simulator. Suspension concentrate formulations were generally retained better than wettable powder formulations. The rainfastness of the fungicides was found to be inversely related to particle size and this partly explained the differences observed between the formulations. Three mm rain applied at a high intensity (27 mm h−1) washed‐off more fungicide than 3 mm applied at a low intensity (3 mm h−1). Addition of various adjuvants significantly improved the rainfastness of the fungicides on both plant species. Oil adjuvants, however, only increased the rainfastness on pea. Furthermore, addition of a latex‐based adjuvant was found to reduce the redistribution of mancozeb on potato leaves.