This study evaluated validity of modified spray decision rules formed to operate axial fan airblast sprayer retrofitted for use in citrus production. The sprayer was field tested in a spraying scenario that involved varied crosswind conditions on small (~ 2 m tall and < 1.5 m wide) and medium (~ 3 m tall and < 2.5 m wide) sized citrus canopies. Crosswinds of 1.3, 2.7, and 4.0 m/s, to the sprayer travel path, were generated using the stationary conical air shaker as the air blower unit. To counter the crosswinds, the amount of air-assist to spray mix was increased and resulting spray coverage along with density of droplets deposited at various locations within both canopy types was evaluated. Water sensitive papers (WSPs) were used as deposit targets and an image processing algorithm developed by Chaim et al. (2002) was used to analyze the WSPs.
Modified variable rate spray decision rules that increased 70-80-100% air-assist for medium sized 2-m tall canopy and 80-90-100% for dense 3-m tall canopy at respective increased crosswind of 1.3, 2.3, and 4.0 m/s were effective in compensating the effect of crosswind. For both types of canopies, spray coverage was higher on canopy front and was decreased as the crosswind counter interaction with spray mix increased. Due to coalescing, larger droplets (Dv,0.5 [volume mean diameter] ~ 1500-2100 µm) were formed on canopy front, whereas coalescing reduced as the droplets penetrated inside the canopy with Dv,0.5 ranged between ~ 600-900 µm on canopy middle and ~ 400 µm on canopy back targets.
