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General recommendations
Economic considerations favor low-volume over dilute sprayers. A low-volume sprayer will cover more acreage in a given time period than a sprayer calibrated for dilute application. However, some tree fruit production chemicals, such as growth regulators and oil sprays, are more effective when applied in larger amounts of water per acre than can be applied with some low-volume sprayers. Check maximum application rate when purchasing a sprayer.
Select nozzle sizes to distribute the spray droplets throughout the air stream. Distribution depends upon tree shape, size, and distance from the sprayer (also row spacing with two-side delivery). Adjustments should be made throughout the growing season and when moving to a block of different size trees.
A wear-resistant nozzle, such as those with ceramic or hardened stainless-steel orifices, should be used. A worn orifice will result in poor droplet formation and distribution. To check orifice wear, mark a section of your orchard covered properly by one tankful of spray mixture when the nozzles are new and the sprayer is calibrated properly. Each time this marked section of the orchard is sprayed, it serves as a check on the flow regulation of the nozzle set in your sprayer. Anytime the variation is more than 5 percent, the nozzles should be replaced. Where whirlplates are used, replace them when nozzles are replaced.
When using a mist sprayer equipped with air-shear nozzles, make sure the fan (engine or PTO shaft) is operating at correct speed. Correct air velocity is very important for proper size of droplet formation as well as tree canopy penetration.
When using PTO-powered units, be sure to match tractor size to sprayer requirements. The tractor must have enough total horsepower to operate the sprayer at rated PTO speed and to maintain a constant ground speed on all terrain conditions. Check sprayer manufacturer's specifications to determine the horsepower requirement of the unit. Be certain the tractor can be operated at the desired ground speed when the engine is operating the PTO at the standard speed. For example, assume a PTO-powered sprayer is operating at 2 mph; the tractor must have sufficient horsepower to provide a ground speed of 2 mph and operate the PTO shaft at the 540 rpm required for proper sprayer operation. Allow 15 to 30 horsepower for transporting the sprayer, depending on its loaded weight and the terrain. Check tractor ground speeds at standard PTO speed to make sure there is a gear that will provide an acceptable rate of travel.
Always check the spray coverage obtained. One rule of thumb is that some spray mist should be 10 feet beyond the tree to obtain complete coverage. Some spray mist should pass through the tree. This is most important when alternate-row spraying is practiced. The effective coverage is less than the extent of visible mist. The mist that carries farthest from the sprayer contains very small droplets and therefore very low amounts of chemical. These very small droplets probably will not deposit on the trees. Techniques using a fluorescent tracer and a black light or water-sensitive paper will provide a more accurate evaluation. The ultimate check is the degree of pest control obtained.