Spray deposits and losses in different sized apple trees from an axial fan orchard sprayer: 2. Effects of spray quality

Abstract

Two field experiments compared spray deposits and losses from very fine (volume median diameter (VMD)=156 μm), fine (VMD=198 μm) and medium (VMD = 237 μm) quality spray applications to young dwarf, dwarf and semi-dwarf apple trees from an axial fan orchard sprayer fitted with hydraulic hollow cone nozzles. All other spray application variables were kept constant. In one experiment, the very fine quality treatment deposited slightly but significantly less spray on leaves than the other treatments but there were no significant differences in the other. The very fine spray quality treatment deposited significantly less on fruitlets in both experiments, but differences were small. The leaf-to-leaf and tree zone-to-tree zone distributions of deposits were not affected significantly by the spray quality. The percentage spray cover, estimated by image analyses, on water-sensitive papers fixed horizontally in the middle of the trees was not markedly affected by spray quality though values were variable. The very fine and fine spray qualities gave significantly greater grand mean percentages of spray drift than the medium spray quality treatment in one experiment, though differences were either not or only just statistically significant in the other. Drift was greatest in the small trees and least in the large trees, consistent with an inverse relationship with tree area density (TAD). The medium spray quality gave significantly smaller deposits on the ground sampling lines than the very fine and fine treatments. In conclusion, variation in spray quality over the range investigated did not greatly affect the amount or macro-distribution of spray deposits on apple trees from the axial fan sprayer. The coarser sprays gave slightly greater mean deposits and smaller spray losses and are preferable from these points of view. Further work is required to establish the effects of spray quality on the micro-distribution of deposits on the target. A coarser pattern of spray deposition on the plant surface could be less efficacious biologically for some pesticides. Therefore, evaluation of the effects of spray quality adjustment on biological efficacy is required.