Orchard Math 101:
For Proper Spray Application
1. Calculate GPA (gallons per acre) for the specific orchard.
Begin with Tree Row Volume (TRV). Refer to diagram on page 3.
TRV = Tree diameter (ft) x Tree height (ft) x 43,560
Row spacing (ft)
Use TRV to determine Gallons per Acre (GPA). A dilute constant* should be selected: 0.5 for bare trees, 0.7 for sparse trees (minimum), or 1.0 for dense canopies (maximum).
GPA = TRV x (dilute constant*)
1,000 cu ft
Proper GPA should give good coverage without over-dosing or under-dosing. Refer to page 3 for recommended adjustments.
2. Select appropriate nozzles to deliver proper gallonage.
Convert GPA to Gallons per Minute (GPM). Note: For tractors without a speedometer, MPH calculation on page 3.
GPM = GPA x MPH x Row spacing (ft)
GPM is the total gallonage needed. Now, divide by the number of nozzles.
GPM per Nozzle = _____GPM_____
Number of nozzles
Choose individual nozzles based on the GPM per Nozzle. Know pump PSI (have a gauge). Hollow cone nozzles are recommended for canopy sprays (airblast). Hollow cone nozzles produce appropriate size droplets for canopy applications. Flat fan nozzles are used for herbicides. Ceramic nozzles are most resistant to abrasion and corrosion. They are worth the extra cost.
See example Tee Jet selection guide below.
Check coverage and drift. Calibrate sprayer. Refer to recommended adjustments section below.
Check coverage, deposition, and drift. Some recommendations include:
ü Deposition should penetrate tree canopy but not blow through to the next row.
ü Monitor deposition with water sensitive paper or photo paper, attach to poles in the center of tree row, stapled upward every 12 inches. Also, use water sensitive paper in next row to monitor blow through and drift.
ü Droplet size should be fine, but not aerosol. If droplets are too big (>300µm), they will bounce from leaf surfaces. If droplets are too small (<150 µm) they will drift without sticking. Hollow core nozzles produce droplets within ideal range.
ü Droplets density (droplets per cm2) should range from 20-30 droplets per cm2 for insecticides and 50-70 droplets per cm2 for fungicides. There should not be streaks or overlapping spots. Refer to label. Sample data sheet from water sensitive paper attached (page 4-7).
ü Adjust nozzle direction for best coverage. Monitor by using poles with flagging (air direction) for even coverage. 70% of spray should cover the top half of the canopy. 30% of the spray should reach the lower half of the canopy.
ü Calibrate sprayers annually by 1) confirming tractor speed (MPH), 2) calculating nozzle output (see page 1), and 3) checking nozzle outputs (see book Effective Orchard Spraying). Error up to ±5%, adjust pump pressure or tractor speed. Error over ±10%, replace nozzles.
ü Most orchard pumps are too large for modern canopy architecture. Ideal pressure is below 150 psi, but centrifugal pumps range from 150 to 200psi. Diaphragm pumps have a wider range of adjustment, but they are more expensive. It is critical to have a gauge to measure psi. Lower pressure by adjusting fan blade pitch, adjusting fan speed via PTO or separate pump. Keep pressure as low as possible; high pressure creates horseshoe vortex and boundary layer.
ü Adjust air volume via reducing intake (wooden donut) or reducing outlet (louvers) to eliminate blow through.
ü Run a constant speed; faster travel speed causes turbulence.
ü Calibrate nozzles and check for wear. Change nozzles if ±10%. Clean tank and nozzles often.
ü Miles per hour (MPH) can be calculated by marking a 100 ft row, using a stopwatch to time the drive run, and then calculating MPH = ___100 ft row x 60__
Seconds traveled x 88
Example: from Syngenta website
What is water-sensitive paper?
Water-sensitive paper is a rigid paper with a specially coated, yellow surface which will be stained dark blue by aqueous droplets impinging on it. It has been developed for field use by Syngenta for the quick evaluation of LV sprays. For droplet assessment, aqueous sprays no longer need the addition of dye. Just place the papers in the target area before spraying. Following exposure to the spray the water-sensitive papers will be stained. Retrieve the papers as soon as they have become dry. Check the droplet pattern. For a quick estimate compare the exposed collectors with a known standard or count the droplets either using a hand lens or an automatic image analyzer.
Water-sensitive paper after exposure. (Courtesy Spraying Systems Co.)
Where to use it
Water-sensitive paper can be used for checking spray distribution, droplet density from aerial and ground spray applications and droplet sizing.
Overdosing is a waste of product. With herbicides, it might result in crop damage and claims. With insecticides underdosing might not kill the pest. Calibrate the sprayer and check the spray pattern. Water-sensitive paper helps you to keep the environment clean.
Airblast sprayers in orchards: staple water-sensitive paper directly onto leaves at the periphery and inside the canopy at the top, in the center and lower part of the trees.
Number the collectors consecutively before placing them on the supports. This will help you to spot irregularities in your spray system when evaluating the exposed cards.
The correct boom height can also be determined with water-sensitive paper. Insufficient overlapping of the spray pattern can be corrected by raising the boom. Excessive overlapping is leveled out by lowering the boom.
Visual assessment of droplet densities
Compare your spray samples with some known standard. The standard cards below and on the following page cover the range of acceptable droplet densities for coarse and medium LV spray. The droplet density in the target area should not be less than:
For routine checking of sprays you might also prepare your own standard cards by selecting spray cards with known droplet densities from previous spray operations.
Standard cards with a known droplet density per cm square
Computer-plotted standard cards displaying the expected number and sizes cards spraying at 3 different volume rates (20, 30, 40 l/ha) and using 3 different droplet spectra (VMD, 200, 300) assuming waters sprayed and the spread factor is two.
This handout was produced for the
Kentucky Extension IPM Implementation Program, Fruit Crops Working Group
By Nicole Gauthier, Extension Plant Pathologist and Ric Bessin, Extension Entomologist.