Rainfastness of selected agrochemicals as affected by leaf surface characteristics and environmental factors

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In our studies the contact fungicide mancozeb and the systemic herbicide glyphosate were used as model substances to elucidate the influence of leaf surface characteristics and environmental factors on rainfastness of agrochemicals. The effect of drying time, rain intensity, rain amount, interruptions of rain showers, and addition of seed oil based adjuvants (rapeseed, linseed, and soybean) differing in degree of ethoxylation were studied in detail in apple seedlings (M. domestica Borkh.). Furthermore, the involvement of surface roughness as well as amount and composition of epicuticular waxes in rainfastness of mancozeb with or without tank-mix adjuvants was examined in adaxial leaf surfaces of apple seedlings, bean seedlings (P. vulgaris L.) and kohlrabi (B. oleracea gongylodes). The interaction between rain intensity and type of linseed oil ethoxylate adjuvant (LSO 10, LSO 0903, LSO 30, and LSO 3003) on the wash-off and biological efficacy of glyphosate was investigated in lambsquarter (C. album), velvetleaf (A. theophrasti) and green foxtail (S. viridis). Light, heavy and torrential rain events with intensities of 0.5, 5, and 48 mm h-1 respectively, were simulated using a laboratory rain simulator. The results can be summarized as follows: 1. Mancozeb was washed-off easily from the leaf surface of apple seedlings due to impact of few millimeters rain, whereas a higher amount of rain caused only little additional a.i. 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, 75 % and 80 % after drying times of 4 h and 24 h, respectively. Intensity and amount of rain independently affected a.i. removal from the seedling leaves. Ten milliliters rain at 0.5 mm h-1, 5 mm h-1 and 48 mm h-1 reduced fungicide concentration of the initial deposit to 43 %, 12 % and 8 %, respectively. Equations for mancozeb removal at light, heavy and torrential rain were determined for precipitation ranges between 0 and 30 mm, and between 0 and 5 mm. Interruptions of rain showers had only little influence on rainfastness at 2 mm rain, and no effect at 5 mm precipitation.