Field evaluation of a tunnel sprayer and effects of spray volume at constant drop size on spray deposits and efficacy of disease control on apple

An air assisted tunnel sprayer (the Noric Joco EX2) incorporating CDA rotary atomisers (Volume Median Diameter (VMD) =c. 140 μm, volume rate = 50 litre ha^-1 forward speed = 3.9 km h^-1) gave at best equal but in some cases significantly poorer control of apple powdery mildew and scab than the standard commercial practice of spraying with an axial fan sprayer with Micron XI rotary atomisers (VMD =c. 90 μm, volume rate = 50 litre ha^-1, forward speed = 7.2 km h^-1). Approximately 30% of the spray volume applied was collected for recycling with the tunnel sprayer. Increasing spray volumes at approximately constant drop size (c. 140 μm) from 50 to 100 to 200 litre ha^-1 with the tunnel sprayer consistently improved efficiency of mildew and scab control. Bulk spray deposits on leaves, and their gross distribution in the tree, was similar with the different spraying methods and volumes. The mean percentage leaf area covered with spray deposit increased with spray volume as did the number of spray deposits per unit area. The tunnel sprayer at 50 litre ha^-1 gave a similar though less variable mean level of cover than the axial fan sprayer at the same volume rate. It gave 2–3 times more cover on upper than on lower leaf surfaces. The axial fan sprayer gave approximately equal cover on upper and lower leaf surfaces. The main limitations of the tunnel sprayer were its slow maximum forward speed and the restricted tree size and shape on which it can be used.