Spray Distribution in a Tree Crop

Abstract

Mature apple trees were sprayed using a Knapsack mistblower fitted with 3 different nozzle systems, one of which provided an induced electrostatic charge on the spray droplets. Spray distribution was assessed by examining fluorescent tracer deposits on Kromekote cards which had been positioned in different parts of the tree canopy. Subsequently, bioassays with codling moth (neonate larvae) and formulations of cypermethrin examined the biological effect of deposits. Small uncharged droplets (30-60um VMD) sprayed at low volume (3.0 4.8 ml/tree) with spinning disc gave the least deposit but with an electrostatic charge, deposition was significantly improved on the outer canopy. Intermediate results were achieved when sprays were applied from an air-shear nozzle at 274.0 ml/tree. Directing spray from both sides of the tree improved spray distribution with all three nozzles. Leaf Area Index (LAI) of the apple varieties used in the experiments was estimated in relation to tree height, and was used to calculate the volumes of spray needed to improve coverage of the canopy. Attempts to improve penetration into canopy were made by varying the charge/mass ratio on the droplets, but a decrease in this ratio reduced deposition (ng/cm2) within canopy. Mortality of Codling moth was related directly to the level of deposits, so there was a decline through the canopy. When large droplets (140-152um VMD) were applied, about 300 droplets/cm2 were needed to deposit 2.75 ng/cm2 active ingredients (ai) to achieve at least 50 percent mortality or more. When smaller droplets (30-60um VMD) were used the number increased to 900 droplets/cm2 to give similar deposits of 4.0 ng/cm2 a.i. and achieve similar responses. Cypermethrin formulations sprayed with each nozzle on apple seedlings (variety Yarlington mill) were more persistent after artificial rain (65.3mm for 15 mins.) when small droplets (VMD - 30.70um) were applied by spinning disc, compared with larger droplets (VMD = 90-169um) produced by the air-shear nozzle. The under surface of leaves were more protected from the direct action of impinging rain droplets, and thus had greater mortality after rain. No significant difference was obtained between droplets by either charged or uncharged spray (P = 0.05).