July 15, 2010
By Dr. Jason S.T. Deveau
Application Technology Specialist, OMAFRA

In June it’s about mid-season for many orchard, high-bush and nursery spray applicators. Some airblast sprayers have already put in over 100 hours. There have been significant increases in canopies since the sprayers were calibrated at the first of the season. Larger, denser canopies mean that it’s time to reconsider spray distribution and sprayer settings. Beyond re-calibrating the sprayer, here are a few important adjustments:

Fan speed

Growers are recognizing that excessive air in early season blows spray straight through the canopy. To correct this, they reduce air speed. This is a good practice. Now, however, canopies are fuller and growers may have to increase the fan speed to compensate. To increase your fan speed, use a higher PTO speed, gear-down and throttle-up, adjust blade pitch or use a hydraulic motor. Remember, you only need enough air to overcome ambient wind and to move leaves and deposit spray in the middle of the canopy. Do not perform alternate row middle spraying.

Nozzle wear

The rate of tip wear depends on spray pressure, the product sprayed and the nozzle material. Upgrading to a harder, more durable tip can reduce maintenance costs, but even ceramic is generally worn in two years. I’ve already pulled disc-core nozzles from sprayers that were new in April, but are completely spent now. How are your nozzles holding up? Even if the sprayer empties where it normally does, plugged filters and strainers can cancel out worn nozzles and you can’t tell with just a shoulder-check. Clean the nozzle strainers and nozzles with a toothbrush in a bucket of water, then check nozzle output one by one. If one is out by 10 per cent, compared to the manufacturer’s rate, replace it. If two are out, replace them all. It’s worth it.

Spray distribution

More canopy often means re-thinking sprayer distribution and output volume. For example, some apple growers choose to open another nozzle position lower on the boom to hit low-hanging branches, but this is not the best way to redistribute spray. The better approach is to turn on a lower nozzle position and then redistribute a higher sprayer output over the entire boom. This way, the whole canopy gets more spray, not just the bottom of the target. Yes, this means replacing all your nozzles with a set that puts out a higher volume, but your nozzles will last longer since you’re changing them mid-season.

Sprayer output

There’s no hard and fast rule, but you should consider increasing overall sprayer output by at least 15 per cent. I was in an orchard early this season, working with a grower to determine an optimal sprayer output. At 600 L/ha, it was over-sprayed. Now, however, the grower needs almost 700 L/ha to achieve adequate coverage in the same trees.

Forward speed

Since you will be reconsidering nozzle rates anyway, also consider slowing down a little to improve canopy penetration. I was visiting a grower last week who was having trouble achieving adequate coverage in the centre of his trees. When we redistributed the spray pattern to match his canopy and slowed down to 5 kph, his coverage improved significantly.

Remember, when you make any changes to your sprayer, put water sensitive paper in the middle of the target canopy about two-thirds from the top (generally the hardest-to-hit location). Spray from both sides and go back to check your coverage. If you don’t see good coverage, further adjustment is required.
Dr. Deveau may be reached at jason.deveau@ontario.ca.