Obtaining the proper distribution of a spray on or within the tree—a concept known as coverage—is essential for good management of citrus pests and diseases. Recommended coverage varies from pest to pest; for instance, an outside coverage spray is best for pests such as citrus thrips and for mandarin rind disorder that are found mostly on the periphery of the tree, whereas pests such as armored scales, which are found deep in the interior of the tree, will need more thorough coverage. For brown rot, skirt sprays may not be adequate for managing Phytophthora species (e.g., P. hibernalis) with detached sporangia and thus, whole tree applications may be necessary. Thorough coverage includes the exterior and the interior of the tree, especially the most difficult to reach top center of the tree. Obtaining the proper coverage can be difficult because of the dense canopy of a citrus tree and involves a balance of varying droplet size, amount of spray, and ground speed (generally, use 1 to 1.5 mph for scale insects, 2 mph for mites, and 3 mph for orangeworms, citrus thrips, and katydids). Spray coverage recommendations are listed as abbreviations in the Treatment Tables for each pest under the Amount Per Acre column. Brief descriptions for each type of coverage are available in the footnotes on each page; the following paragraphs give a more detailed description of the various types of coverage, starting with the lowest gallonage applications.
Aircraft applications typically use 5 to 20 gallons of water per acre. For fungicide applications, higher volumes of 20 to 30 gallons of water per acre are generally needed.
Low-volume applications generally use 100 gallons of water per acre, but not less than 20 gallons per acre. To be effective and avoid phytotoxicity with oils and some chemicals, low-volume applications must be delivered in small drops having a median volume diameter of less than 50 to 150 microns. This droplet size can be obtained by providing air velocities in excess of 175 miles per hour (mph) at the discharge outlet if non-atomizing spray nozzles are used, or by utilizing spray pressure in excess of 250 pounds per square inch (psi) if atomizing nozzles are used.
Low-volume applications use between 75 and 100% of the amount of pesticide active ingredient that would be applied per acre as a dilute spray. An exception is narrow range oil for mite control: it may be used at 10 to 15 gallons per acre plus water to make 50 to 100 gallons of spray per acre.
In order to achieve proper coverage, apply low-volume sprays only when it is relatively calm. Never apply low-volume sprays if wind speeds exceed 5 mph. In addition, extra precautions must be taken to protect handlers and applicators because of the high concentrations of chemicals used in low-volume applications.
Outside Coverage (OC)
Outside coverage applications use 100 to 250 gallons of water per acre and ground speeds of 2 to 3 mph to spray the outside parts of the tree only. Larger trees and high-density plantings require the higher water volume per acre. Outside coverage is used to control pests that prefer the exterior leaves and fruit of the tree such as aphids, mites, citrus thrips, and several species of orangeworms.
Intermediate Coverage (IC)
Intermediate coverage uses 250 to 600 gallons of water per acre. Intermediate coverage is used for pests such as citricola scale that reside primarily on the leaves but infest both interior and exterior leaves. Similarly, spray penetration is needed for some diseases such as anthracnose and Septoria spot that reside on inner leaves and twigs. Therefore, increased water volume and slower spray rig speed (1–1.5 mph) is needed to penetrate the first layer of leaves.
Thorough Coverage (TC)
Thorough coverage uses 750 to 2,000 gallons of water per acre, depending on tree size, to achieve a film wetting on all interior and exterior parts of the tree (leaves, twigs, and fruit). Large trees may require spraying from a tower to obtain suitable coverage in the top part of the tree. Ground speeds should be less than 1.5 mph, pressures in the range of 450 to 550 psi, nozzle adjustments to provide a spray cone angle of less than 16 degrees. In addition, for oscillating boom sprayers the rate of boom oscillation should be between 62 to 70 oscillations per minute. With air blast sprayers, faster ground speeds will cause a sheeting action of the leaves that will actually reduce penetration and result in poor coverage. Generally thorough coverage is used with all sprays targeted against armored scale insects. No pesticide requires more complete coverage than petroleum spray oils, because oils act to smother the pest or render the surface of the plant unusable by pests.