Adjusting Airblast Sprayers for High-Density Apple Orchards)

Dr. Jason S.T. Deveau, Application Technology Specialist, OMAFRA

As Ontario continues to plant high-density apple orchards (see Figure 1), spray operators are faced with the challenge of adjusting their spray practices to match the size, shape, area-density and stage of growth of the trees. The same issue was encountered in the seventies when growers moved from standard apple trees to semi-dwarf varieties.

Back then, growers used a pro-rata formula called Tree-Row Volume to reduce their output from ~3,750 L/ha (400 US g/ac) to approximately 1,000 L/ha (~110 US g/ac). The method worked, and todays’ agrichemical products developed for use in apple orchards are generally tested in 1,000 liters of spray volume per hectare (with the exception of products intended to drench the target).

A new model is required that will allow the applicator to maintain (or improve) insecticide and fungicide efficacy while minimizing waste. The Crop-Adapted Spraying (CAS) model currently in its third year of testing by OMAFRA is based on the premise that pesticides sprayed onto crops like tree fruit should not have rates that reflect the area of the planting. They should reflect the tree area-density, which means the amount of crop in the orchard, not the area of the orchard planting.

The ultimate goal of CAS is to adapt the amount of active ingredient per unit ground area such that the amount of active ingredient per unit target area (usually the leaf area) remains constant for canopies of varying shape and density. When this is achieved with sufficient accuracy, the pesticide efficacy is maintained.

Here are the steps to follow if you are spraying a young and/or high-density orchard with an airblast sprayer:

Sprayer should receive all seasonal maintenance prior to first use and undergo a visual inspection before each spray day. Park sprayer in an alley between rows of trees and tie 25 cm lengths of ribbon to the ends of the deflectors (if present) and the nozzle bodies. Turn on the air and extrapolate where the nozzles and deflectors are aimed. Adjust deflectors and turn off nozzles that will spray over or under the tree canopy (See Figure 2). Consider using air-induction hollow cones in the top positions of each boom to reduce drift. You may have to increase the rate in those positions to compensate for the fact that nozzles producing larger droplets produce fewer droplets. Confirm ground speed with a half-full sprayer in the orchard using GPS or a calibration formula. Affix 25 cm ribbons to far side of three trees. Tie them at the top and at the widest portions of the canopies. Drive past in the spraying gear at the ideal RPMs with the air on, and ensure the ribbons waft outwards. This will determine if more/less air is required from the airblast sprayer, and if operator should speed up or slow down during spraying (see Figure 3 and Figure 4). This is also an opportunity to perform Gear-Up-Throttle-Down if sprayer is using a positive displacement pump. Watch “How to calibrate air on an airblast sprayer” for more information: https://www.youtube.com/watch?v=u8wNd2NREW4 Place water-sensitive paper at the top, centre and bottom of the tree canopy and spray water (see Figure 5). As an approximation, if coverage exceeds 15% surface-area and 85 discrete droplets per square centimetre, reduce output in those positions by replacing nozzles with lower outputs. If less than ideal coverage is achieved, increase the nozzle rates in those positions. Excessive coverage may be unavoidable in the outer edge of the canopy, given that spray must pass through to get to the centre. Be aware that ambient wind speed and humidity have significant impacts on coverage. Therefore, only test coverage in conditions similar to your typical spraying conditions. Watch “How to use water sensitive paper” for more information: https://www.youtube.com/watch?v=P79fH2VdJic When the canopy grows and fills in sufficiently, usually after petal fall, repeat steps 4 and 5. If you are suspicious that the spray is being stretched too thin or you are unsatisfied with the coverage, you may have to increase the output. This is more of an issue with larger trees. Early in the season, wind travels relatively unimpeded in a high-density orchard and will blow the spray off course, reducing coverage and requiring higher water volumes or possibly more air to compensate. As the trees fill in, the average wind speed is reduced and more spray can impact on the target. Therefore, increasing spray volume after petal fall may not be required in a high-density orchard.

When the correct sprayer settings and volumes have been determined, the operator will mix their spray tank as they would for their typical application to a semi-dwarf orchard. However, they may be spraying as little as 300 L/ha in a high-density orchard, and will therefore go further on a tank (see Figure 6). The leaves will receive the same absolute amount of active ingredient as in a larger, denser orchard; the difference will be that the spray is not wasted through over-spraying.

This method of application is really no more sophisticated than turning off nozzles that are aiming at the ground or above the target. It will take time for operators to get comfortable with the new volumes (and reduced dosage per hectare) and regular scouting is highly encouraged to confirm they are achieving control.