Web site - Tree Spraying

Tree Spraying

The air-blast sprayer plays an important role in achieving the level of pest control obtained with a specific amount of pesticide. Best results are obtained when the sprayer has enough fan capacity to blow the spray through the trees and at least 10 feet beyond, even when operating against a 5-mph wind. Maximum spray deposit requires that the droplets be forced against the object to be covered. Spray that drifts at slow speeds past tree tops is not sufficient.

Many sprayers are unable to achieve adequate deposit on trees over 20 feet high. Most sprayers should be operated at 2.5 mph or less on mature trees. When low air volume sprayers are being used, the ground speed usually must be limited to 2 mph or less, even on trees of small to moderate size. Sprays should be applied only when there is little or no wind. Large trees require sprayers with large air volume capacities. Match the sprayer capability to the tree size. Air capacity and air speed are not the same. Use water-sensitive paper targets in the trees to evaluate coverage.

Low-volume (concentrate) spraying involves reduced amounts of water per acre, generally a reduction from 350 to 400 gallons per acre for dilute sprays to 20 to 100 gallons. The term "low volume" is derived from the fact that a smaller volume of water, not air, is used to carry the chemical. Runoff is eliminated with low-volume spraying. Individual sprayers are designed to operate most efficiently at certain gallonages per acre, and best spray coverage and deposit are obtained within the manufacturer's recommended range. Tree size and number of trees per acre as well as spray droplet size influence the gallonage needed for adequate coverage.

Choose the gallonage per acre best suited to your equipment, tree size, and orchard problems. Then add the amount of chemical needed per acre to that amount of water. Spraying less than 40 gallons of water per acre onto trees over 18 feet high usually results in unsatisfactory coverage. See Tree Row Volume in this section.

The amount of pesticide per acre in low-volume spray is reduced in comparison with the amount needed in a standard 400 gallons of dilute spray per acre. For example, a fungicide might be suggested at 8 pounds in 400 gallons of water per acre. With low-volume sprays on apples and sweet cherries, the 8 pounds can be reduced about 20 percent. In low-volume sprays for peaches, pears, nectarines, plums, and tart cherries, the rate can be reduced by about 25 percent or to 5.25 pounds per acre. For lower rates to be effective, the entire tree must be covered without runoff.

The advantages of low-volume spraying are: less pesticide, water, labor, and time, with fewer refills. The disadvantages are in the increased care required to calibrate the sprayer, maintain a constant ground speed, select good spraying conditions, and train a skilled operator. As gallonage is reduced, errors become more critical. In addition, some materials such as dormant oil and growth regulators need to be applied at higher water gallonage per acre to be effective. Rates of water of 100 to 300 gallons per acre may be required. Fire blight sprays should be applied at full dilute rate. Dual "flop over" nozzles, multiple-orifice nozzles, or adjustable flow to air-shear nozzles are convenient time savers.

Application costs decrease most rapidly when changing from 400 gallons to 50 gallons of spray solution per acre. Below 50 gallons per acre the savings are smaller, and complete coverage will be difficult even on moderate-size trees. The additional savings with less than 50 gallons per acre may not be worth the additional risks.

Alternate middle row spraying often is used by fruit growers. The basic plan calls for one complete coverage of apple trees before the first scab infection period in the spring. Then alternate rows are sprayed at a fixed interval, such as every 7 days. This method permits frequent application of low amounts of pesticides, many of which are short-lived on the tree. With minor modifications, it is the preferred method in pest management programs and in other programs where minimum use of pesticides is a major goal. If the advantages of the alternate-row spraying method are to be realized, it is essential that the sprayer have adequate air volume and velocity to provide at least light spray coverage on at least 90 percent of the tree each half spray. With full-size apple trees planted in rows 30 to 40 feet apart, large sprayers delivering 90,000 to 100,000 cubic feet of air per minute at 80 mph or more are satisfactory for alternate-row spraying. Smaller trees and closer rows permit sprayers with smaller air capacities to be used. For good results, the sprayer should push some spray 10 to 15 feet beyond the tree.