Spray deposits and losses in different sized apple trees from an axial fanorchard sprayer: 1. Effects of spray liquid flow rate

Spray deposit and off-target contamination distribution measurements were m ade in apple trees of different sizes and row spacings for an axial fan spr ayer with different spray liquid flow rates (c 3.8, 11, 291min(-1)) but oth erwise constant conditions (air flow = 11.3 m(3) s(-1); speed = 5.8-5.9 km h(-1), VMD = 156-159 mum). Leaf deposits (normalised by the applied dose) w ere not affected by the spray liquid flow rate except on young dwarf trees at the high flow rate due to saturation. The volume retained on the crop in creased with tree size. Deposits on individual leaves were approximately lo g normally distributed. Variability was usually marginally greater for the low flow treatment than for the medium or high flow treatments and usually greater in the larger than in the smaller trees. Mean deposits on the under sides of leaves were about double those on upper surfaces. The mean percent age of the surface area of water sensitive papers in the centre of the tree covered with spray varied in direct proportion to the spray liquid flow ra te. Spray cover was least in the larger trees. Deposits on fruitlets were a ffected significantly by spray liquid flow rate. The high flow rate treatme nt gave the smallest deposits on fruitlets due to the deposit exceeding loc al saturation conditions. The deposits on fruitlets tended to be greater in the smaller than in the larger trees. Normalised spray drift deposits on v ertical, 10 m tall polythene sampling lines positioned 5 m downwind of the sprayer were somewhat greater for the low flow rate than the medium and hig h flow rate treatments but differences were at least partly due to differen ces in wind direction. High values occurred where the wind direction was pe rpendicular to the tree rows, low values where the wind direction was paral lel to the tree rows. Drift generally decreased with increasing tree size. There were no consistent differences in normalised ground deposits due to s pray liquid flow rate, though ground deposits were greater in the smaller t rees than in the larger trees. As a whole, these results showed that wide v ariation in spray liquid flow rate had only small effects on normalised spr ay deposits on the tree, on spray drift and on losses to the ground but gre atly affected spray cover which varied in proportion to the spray liquid fl ow rate. This implies that the spray liquid flow rate for foliar applicatio n of an agrochemical should be determined by the degree of spray cover that is needed for adequate efficacy. Furthermore, the pesticide dose rate may be conveniently adjusted in direct proportion to the spray liquid flow rate at constant concentration for deposits lower than the target saturation le vel. (C) 2001 Elsevier Science Ltd. All rights reserved.