Mancozeb wash-off from apple seedlings by simulated rainfall as affected by drying time of fungicide deposit and rain characteristics

In our studies apple seedlings (Malus domestica Borkh.) were used as model plants to elucidate the influence of drying time of the deposit, rain intensity, and rain volume on rainfastness of the contact fungicide mancozeb. Light (0.5 mm h−1), heavy (5 mm h−1), and torrential (48 mm h−1) rain events were simulated using a laboratory rain simulator. In a first experiment, wash-off of mancozeb independent of drying time (2, 4, and 24 h) was evaluated after 5, 10, 20 and 30 mm heavy rain. Results show that mancozeb was washed-off easily from the leaf surface of apple seedlings due to the impact of a few millimetres rain, and a higher volume of rain caused little additional removal. Regardless of drying time, fungicide removal from the leaves followed a hyperbolic curve. Fungicide losses after 5 mm rain reached about 90% of the initial deposit after a drying time of 2 h, and 75% and 80% after drying times of 4 h and 24 h, respectively. In a second experiment light, heavy and torrential rain events were simulated and sequential samples were taken after 1, 2, 3, 4, and 5 mm precipitation. Regardless of rain intensity, fungicide removal was observed after 1 mm rain (9% for light rain, 55% for heavy rain, and 80% for torrential rain). Fungicide wash-off followed a hyperbolic curve in the cases of heavy and torrential rain, and a linear progression at light rain conditions. After 5 mm rain, losses amounted to 90% under heavy and torrential rain, and 50% under light rain. Our data show clearly that wash-off of mancozeb deposits is influenced by both rain volume and intensity.