Program Contact: Tianna DuPont, Regional Specialist, Tree Fruit
(509) 663-8181 •

Timothy J. Smith, Washington State University Area Extension
Chelan, Douglas & Okanogan Counties

Each season, an orchard air blast sprayer may apply materials worth from $350 to $1300 per acre. Many of the materials applied have the potential to protect the fruit and tree from pests, provide nutrition, or regulate the tree or fruit growth. However, the failure to apply a spray evenly to its target can cost the grower much more than the original price of the material and the $21/acre application costs. Poor spray coverage, especially on larger trees, is the primary cause of reduced spray product performance.

Variations in tree structure, the time of season, and the purpose of the spray being applied require you to frequently adjust the sprayer output and vary the application speed as the growing season progresses. These adjustments are often bypassed because sprayer calibration and adjustment is considered difficult and time consuming. Surveys have shown that the average small acreage grower could save over $1,500 per year in spray material costs alone by investing a few hours and dollars in maintaining and adjusting their sprayers. Material and labor savings, combined with improved affect of sprayed materials will greatly reward the grower for the time spent in following the steps outlined below.

Step 1:

Determine the proper speed of travel.
Driving the sprayer faster than the conditions allow is the most likely cause for poor coverage. A careful evaluation of sprayer speed is most likely to make a major difference in spray coverage and spray material performance. It takes time for the air and spray product mixture to completely penetrate the tree volume and contact all surfaces. The amount of time necessary increases with increasing tree size and density. As the growing season progresses, leaf and shoot growth adds to the density of the tree. For any given sprayer, you must drive progressively slower as the season advances.

triangleThe sprayer must travel at the speed that provides for good coverage of the upper central portion of the tree. With most sprayers, this means from 1.0 to 2.5 mph., depending on tree structure. How can you tell when the speed is right? Using plain water as a test product, you can apply at various speeds until a visual check shows you that there is adequate coverage of the difficult to reach areas of the tree. Record the gear and rpm used when coverage of the upper central part of the tree is good.

Then measure the number of seconds it takes to drive 88 feet at this speed. Divide the seconds into 60. This is your miles per hour. Do not trust your tractors speedometer until you have verified it’s accuracy. You will need this accurate measurement of mph to know or properly adjust the sprayer’s per acre application rate.

With engine driven sprayers, the speed can be adjusted relatively easily. With power-take-off driven sprayers, you must select the speed based on the gear and rpm range that properly powers the sprayer relative to the tractor speed. This ground speed may happen to be slower than you would like to drive, but proper coverage is much more valuable than time/labor costs. Note the table below:

Minutes Required to Spray 1 acre, not counting time necessary to turn at the end of rows:


Miles per hour
Tree Row Spacing (feet)


Example: Your present speed is 2.2 miles per hour, and you are spraying pears or cherries growing at a 20 foot row spacing. The next gear down at the same engine RPM reduces your speed to 1.8 MPH. Instead of the 11.3 minutes of actual spraying time per acre at the higher speed, it will take 13.8 minutes. So spraying at the lower gear will cost you 2 minutes, thirty seconds per acre which adds a few dollars to your machine operation cost (2.5 minutes of fuel, repair and labor costs), but may greatly improve the effect of the expensive products you are applying, perhaps improving the value of your fruit.

Studies have found that as much as half of spraying time is actually spent driving back and forth to the fill station and mixing/.loading the sprayer. If you want to spend less time spraying, working to reduce this non-spraying time part of the operation is often a better choice than driving faster while spraying.

.Now that you have chosen a speed that will give you good coverage, you may set up your sprayer to apply your preferred gallons per acre rate. The gallons per acre you choose should vary with tree size and the purpose of the product you wish to apply.

Step 2:

Adjusting gallons applied per acre.

Tree surface coverage is only slightly influenced by the gallons of spray mixture applied per acre, but the type of coverage is greatly affected. Poor coverage is possible with any gallonage. Low volume applications apply distinct droplets, while high volume sprays apply a sheet of water and material to the tree surfaces. Some products must completely wet the fruit or leaf surface for best effect. Also, some pests may be protected behind bark scales or pruning stubs on older trees, so high volumes of water may be necessary to move the material around after it contacts the tree. The amount of water that is necessary to spray a tree at dilute (or to drip) depends on the tree size and foliage thickness. In Washington, the amount of water per acre that constitutes “dilute” or “full coverage” varies from 60 to 600 gallons. The amount of water per acre necessary to have the same degree of tree coverage increases rapidly as the orchard develops its foliage.

Medium or low volume sprays should be adjusted to the gallonage that allows the majority of the tree surface to be sprayed with distinct small droplets, but with little movement of the spray material after it contacts the foliage and fruit surface. You must avoid applying concentrated materials to the tree at gallonages that allow the material to dribble to the lower surface of the fruit. Evaporation of the water carrier could concentrate the resulting drip and lead to fruit marking. With our large trees, between 100 and 200 gpa tends to be the most hazardous to the fruit.

To set up the gallons per acre, first determine the gallons per minute that each side of the sprayer must apply.

Multply Gallons per acre desired by Miles per hour.  Then multiply that number by the feet spacing between rows.  Then devide the resulting number by 990.  The result is the gallons that should be sprayed each minute by each side of the sprayer.

Example: You wish to apply 80 gallons per acre, there are18 feet between rows and you get best coverage at 1.8 mph driving speed.

     80 gallons,  multiplied by 1.8 mph,  multiplied by 18 divided by 990 equals 2.62 gallons each minute from each side.

abSTEP 3:

Setting up the sprayer nozzle manifold.
Park your sprayer in a typical part of your orchard. Stand behind it and draw imaginary straight lines from the center of the fan through various parts of the tree next to the sprayer. The spray traveling in a straight line through the tree in the directions marked “A” in the figure to the right will have much less surface to cover than spray moving in the direction marked “B”. Note which nozzles are in direct line between the center of the fan and various portions of the tree.

NOZLPLACWhen arranging the nozzles along the manifold, you must purchase a mixture of nozzle sizes that, when totaled, put out the proper gallonage per minute for each side of the sprayer. Then place the nozzles on the manifold in relation to the tree size and shape. Usually the largest nozzles are in line with the thickest portion of the tree, with medium sizes and smallest sizes arranged so that gallon per minute output tapers off on either end. If the sprayer has more than two manifolds, the principle remains the same; output per minute should be highest along the portion of the manifold in line with the bulk of the tree. Sometimes the highest and lowest nozzles on the manifold are not used, especially if the trees are large and row spacing is relatively wide.

Example from Above: You wish to apply 80 gallons per acre, there are18 feet between rows and you get best coverage at 1.8 mph driving speed calls for 2.62 gallons of spray per side per minute. Using a nozzle chart you choose 7 nozzles, 3 small, 2 medium and 2 large, with a total out put per minute close to this amount.

Nozzle and Maintenance Test:

Now that your sprayer is approximately set up for your spraying conditions, follow the exercise outlined below, then record the size and position of the nozzle discs, the core sizes, the speed of travel (gear and rpm for the tractor you used), and the total gallon per minute output. You will need this information regularly when you re-check the sprayer. Hang it on the wall of the spray shed, or somewhere handy.

  1. Fill sprayer with water to overflowing.
  2. Without moving the sprayer, run both sides for 3 minutes at operating pressure (2 minutes if spraying dilute).
  3. With a calibrated bucket, refill the sprayer to measure gallonage sprayed. 
  4. Divide gallonage sprayed by 6 to determine the output per side (divide by 4 if the sprayer was run only 2 minutes).
  5.  Compare actual sprayer output with calculated output (from Step 2 of this article). If necessary, alter pump pressure slightly to adjust sprayer output. Sprayer pressure should be opperated near the middle of the suggested psi range, so spray consists of the proper range of particle sizes.
  6. Record pump and manifold pressure necessary to operate new nozzles and cores at the correct output per minute. (Have you had the pressure gages checked lately? They tend to become inaccurate after a few years).
  7. Recheck your gallons/minute output regularly. If the output rises by 5% at a constant pressure, the nozzles are wasting more money than it takes to replace them, and are probably putting out a less than optimum mixture of spray particle sizes.

Nozzle wear occurs most rapidly when wettable powders, flowables, or dispersible granules are applied, especially at high nozzle pressures. Under these situations, the tips and cores on the sprayer should be manufactured from hard, wear resistant materials. The abrasion resistant nozzle components cost more initially, but in the long term are quite cost effective. The first table illustrates the relative wear rate of various nozzle materials. As you can see, brass wears at a rate 10 to 15 times faster than hardened stainless steel. The second table shows that the more wear resistant nozzles have a higher price, but are more cost effective.

Nozzle Material Relative Durability

 Brass  1
 Stainless Steel  3 – 4
 Hardened Stainless Steel  10 – 15
 Tungsten Carbide/Ceramic  80 – 150

Example: Ceramic will last up to 150 times as long as Brass under the same spray conditions.

Nozzle Tip Material Initial Cost and Cost per Unit of Wear (example)



 Nozzle Material  Per Nozzle Cost (Example)  Cost Per Unit of Wear
 Brass  $4.00 $4.00
 Satinless Steel  $8.00 $2.00
 Hardened Stainless Steel  $12.00 $0.80
 Tungsten Carbide/Ceramic  $36.00 $0.24

Useful Calibration Formulas

A. To determine the total gallons per minute that should be sprayed from both sides of the sprayer. (see Step 1 & 2 of this proceeding article).

Desired gallons per Acre x MPH x Row Spacing (feet)
___________________________________________________________  =    Sprayer output per minute
Divided by           495

B. If you know the total gallons per minute output of the sprayer and have an accurate measurement of the speed of travel and wish to determine the gallons per acre output:

Gallons per Minute Sprayer Output  X  495

                    _____________________________________________________   =  Gallons per Acre

                                                 Tree Row Spacing x Miles per Hour

C. If you know the gallons per minute output of the total sprayer and the gallons per acre you wish to apply, and want to know the speed that you need to drive (note: check the coverage at the given speed!)

Gallons per Minute Sprayer Output x 495
________________________________________________  = MPH  you drive

Desired gallons per acre x tree row spacing

D. To accurately determine your speed in miles per hour, time equipment over an 88 foot length. Divide the seconds elapsed into 60.

________________________________ = MPH
Seconds it takes to drive 88 feet

Washington State University