Air-assisted sprayer adapted for precision horticulture: Spray patterns and deposition assessments in small-sized citrus canopies

An axial-fan air-assisted sprayer, adapted with variable rate nozzles and adjustable air-assist flow control, for citrus tree-specific precision spraying, was tested. Static experiments were conducted to determine spray patterns at varied nozzle flow rates and air-assistance settings. Spray patterns confirmed that the sprayer delivered variable liquid flow rates and the air-diverting louvre controlled the air-assistance providing targeted spray delivery. Field experiments were conducted to determine suitable air-assistance and nozzle combinations needed in the future for sensor-controlled spraying in young (small-sized) citrus canopies. Experiments involved two nozzle treatments, a) nozzles 1–6, and b) nozzles 2–3 at 100% output rates and three (40%, 70%, and 100%) levels of air assistance selected based on spray patterns. The sprayer was operated at a travel speed of 4.5 km h−1 and the water-soluble tracer deposition on biological and artificial targets, at 0.6 m and 1.4 m above ground and laterally at the canopy front, middle, and back, respectively were measured using fluorimetry. Results suggested that within nozzles 1–6 treatments with varied air-assist, 70% air-assistance was more effective for small sized canopies than 100% air-assistance; it appeared that the latter propelled the spray beyond the canopy and reduced the spray deposition efficiency. Also, for the citrus canopies studied, the use of two or three nozzles instead of the lower six nozzles (i.e. the control treatment) might be more suitable since they would result in 50% or less chemical usage while having comparable spray deposition to that of the control.