Modeling losses of copper-based fungicide foliar sprays in wash-off under simulated rain

Abstract

Wash-off experiments of three Cu-based fungicides were conducted with a single raindrop simulator with known drop size and fall height. Losses were quantified as total Cu (Cu T ), in solution (Cu L ), and particulate (Cu P ). Cu wash-off time course was modeled for two different drop sizes using a stochastic model based on the cumulative detachment by random scattered raindrop impacts. In other set of experiments, the influence of raindrop size, fall height, and fungicide dose was analyzed statistically by means of a full factorial design. Most Cu was lost as particles sized from 0.3 to 1 lm. The stochastic model gave good estimations with two detachment performance levels. The best-fitting model parameters were as follows: the single element area covered by one impact (1.7 and 0.38 cm 2 for the large and small raindrop, respectively), the average number of repeated drop impacts on one single element area that exhaust non-rainfast fungicide (4.2 ± 3.0 small drops and 2.5 ± 0.5 large drops for the high performance level, low performance level needed 30 ± 10 large drops and 40 ± 88 small drops), and the mass washed-off by a single-drop impact (from 1.27 ± 0.2 lg Cu to 3 ± 1 ng Cu per impact). Factorial design showed the dosage was the most influential factor in the three fungicide formulations. The model can help to estimate fungicide losses in field from rainfall and canopy properties. However, the particulate/soluble loss ratio cannot be predicted by the model since both the particle detachment and solubilization were not clearly related with the raindrop energy.

Modeling raindrop strike performance on copper wash-off from vine leaves

“…The Cu P and Cu S losses found in this study were similar to the losses reported in wash-off experiments conducted with artificial substrates and potato leaves (P erez- Rodríguez et al, 2013aRodríguez et al, , 2013b. In general, the model reproduced the temporal loss of Cu wash-off quite well, but it did not accurately reproduce the loss of Cu P .…”

“…The velocity of each raindrop at the point of impact and the kinetic energy were calculated for raindrops falling from 1-and 2.5-m heights (Epema and Riezebos, 1983) and they are shown in Table 1. An equivalent diameter was also calculated for the same fall heights by P erez- Rodríguez et al (2013a). These raindrop sizes are characteristic of raindrops found in moderate to heavy rains (Byers, 1959;Pruppacher and Klett, 2010;Rogers and Yau, 1996).…”

“…The simulator used in this study was set-up following P erez- Rodríguez et al (2013a). A robotic rainfall dripper provided a uniform random scattering of raindrops within a circle that was 4 cm in diameter.…”

“…This amount was tested previously in the temporal experiments to guarantee that pesticides were not exhausted in the leaves. The mass balance in a steady state wash-off avoids the effect of exhaustion of the Cu after large episodes, which could mask the influence of other factors on the loss of Cu (P erez- Rodríguez et al, 2013aRodríguez et al, , 2013b. A two-factor by two-level full factorial design with three replicates was carried out to examine the influence of S and H for each fraction of Cu (Cu S , Cu P and Cu T ).…”

“…Hydraulic shear stress tests performed on surfaces sprayed with a Bordeaux mixture and a copper oxychloride-based formulation produced pesticide washoff losses that were in both particulate and soluble forms (Paradelo et al, 2008); the particulate losses were greater than the soluble losses. This finding led P erez- Rodríguez et al (2013a) to examine the factors that influence Cu-losses produced by rainfall. They used a single raindrop simulator to examine rainfall on synthetic surfaces.…”