Reducing spray volumes applied to mature citrus trees

Maximum retention of spray and run-off volumes on citrus leaves were studied in a laboratory experiment and then compared with a field trial. In the field trial an air-assisted sprayer fitted with a tower air conveyor was used to apply between 1000 and 8000 l ha-1, atomised as a uniform spray spectrum, at different forward speeds. The volume of spray retained by leaves increased with increasing application volumes but the percent retention decreased above an application volume of 2000 l ha-1. The proportion of spray lost as canopy run-off also increased above 2000 l ha-1. Spray deposition was highest on leaves in the bottom tree zone, decreased in the top tree zone and was lowest in the middle tree zone. This deposition pattern is probably caused by the airflow pattern generated by the sprayer and indicates that uneven distribution of airflow to the tree is related to uneven leaf coverage throughout the canopy. A tractor speed of 3.6 km h-1 did not reduce leaf retention of spray volume compared with the slower speed (1.7 km h-1) but reduced canopy run-off at the 4000 l ha-1 application volume. Canopy spray run-off was highest along the tree axis parallel to sprayer travel and lowest along the tree axis perpendicular to sprayer travel. The application of pesticides to citrus at volumes that do not exceed 2000 l ha-1 reduces the loss of pesticide as canopy spray run-off and maximises pesticide spray recovery on the citrus leaf canopy