Getting the Best Spray Coverage from a Hull Split Spray

Franz Niederholzer, UC Farm Advisor, Colusa/Sutter/Yuba Counties  Hull split is not far off. Where needed, an effective hull split spray can be the difference between good and bad (costly) reject numbers. Spray timing and pesticide selection are important parts of a good worm (and mite) control at hull split, but so is spray coverage. A bad spray job can waste time and money. In recent research conducted by Dr. Ken Giles (UC Davis Dept of Ag and Biol Engineering) and funded by the Almond Board of CA, hull split spray coverage and navel orange worm control was less in the tops of mature almond trees compared with that in the lower canopy. Here are a few things to consider when setting up your sprayer(s) for hull split spraying that should help improve spray coverage in the tree tops.Airblast sprayers are air carrier sprayers. The pesticide droplets go where the air from the sprayer’s fan goes. Increasing the spray volume per acre will not change coverage in the parts of the canopy – usually the tops – where sprayer air doesn’t reach. How can a grower make sure that good spray coverage is being obtained throughout the tree? Usually this means driving slow, giving the air from the sprayer fan time to reach the tree tops. How slow? Slow enough to give good coverage in the tops of the trees. In 2011, Dr. Giles and his team found that 2.0 MPH compared to 2.5 MPH sprayer speed significantly improved navel orange worm control at hull split. Slower was better. Not sure what speed to drive in your orchard(s) with your sprayer(s) for the best spray coverage? Check ground speed and spray coverage in the orchard just before hull split. Here are some examples of how to do this:Watch the sprayer air: Using a pruning tower, get up in the tops of some representative Nonpareil trees and tie surveyor tape to the highest shoots – the toughest target in the tree. Then have someone drive the tractor down the row spraying water using the sprayer set up that you expect to use at hull split while you are watching the tree tops from a couple of rows over in the pruning tower. Does the surveyors tape move at all when the sprayer drives by? It should. If it doesn’t move much or not at all, then slow the tractor down and do it again. You want the tape to wave gently as the sprayer goes by, not hang limp or stand out like a flag in a hard wind. [If the flagging stands out straight, you are driving too slow, moving too much air per tree and wasting time and money.] Don’t have/can’t borrow a pruning tower? Tie surveyor tape to the tops of several long PVC poles and stand the poles upright in the interiors of several trees in a row. Tie the poles to scaffolds or branches to hold them steady. The tops of the poles (and the tape) should be at or near the tops of the highest shoots. Watch the flagging from a distance and see if/how much movement there is. Thanks to Kim Blagbourne at Slimline Manufacturing for information on this simple and effective test.Watch the spray: Spray the trees with clean water or water + clay product (Surround™, etc.). As soon as the sprayer goes by, use the pruning tower to get up in the tops of the trees and see how the coverage looks. Not enough coverage? Slow down. Plenty of coverage (spray material pooling on the lower edges of the leaves and nuts)? Speed up. Another option is to put water sensitive paper in the tops of the trees, or at different locations in the tree canopy from top to bottom, then run the sprayer down the row. Adjust sprayer speed and nozzle size placement so the spray droplet patterns are as uniform as possible on cards spaced evenly from bottom to top of the canopy. Again, if you don’t have access to a pruning tower, attach the water sensitive cards to PVC poles so they can be placed at different locations in the trees – especially the tops. Note: Also check coverage on the leaves and nuts in the lower part of the canopy while you are making sure the tops get good coverage. If there is too much coverage in the lower canopy (spray pooling on the lower edges of the leaves, spray cards solid blue with spray drops on the lower edges of the cards), you might consider putting smaller nozzles on the positions on the spray manifold that target the lower portion of the canopy and shifting more spray flow to the upper nozzles.Finally, if you change sprayer ground speed based on any of the suggestions above, remember to carefully measure the new ground speed and recalibrate the sprayer so you still deliver the gallons per acre spray volume used to determine the amount of pesticide to add to each tank. You may end up accidently changing the rate of pesticide sprayed per acre if you change the ground speed without recalibrating.