Back to March 2021

Make the most of your airblast sprayer, your most valuable orchard asset

An airblast sprayer sprays young pecan trees in an orchard in Texas.

Zinc spraying was underway in early April at Swift River Pecans during the 2017 crop year. (Photo by Mallory O'Sullivan)

For anyone attempting to grow pecans for profit, the only essential piece of equipment for accomplishing that task is an airblast sprayer. Well, you also need a tractor to pull it through the orchard and power the pump on PTO-type machines, so that’s really two essential pieces of equipment. Every other piece of equipment used in a pecan orchard is a luxury of sorts.

Yes, that’s right. With enough cane poles, buckets, and people, you don’t need a shaker, harvester, or cleaner. Sure, they help expedite harvest and decrease man-hours and harvest costs, but that equipment is not essential to being in the pecan business. Take away a functional and well-performing airblast tree sprayer, and any would-be pecan farmer’s assurances of producing a crop go out the window!

Without ground-driven airblast sprayers, originally called “speed sprayers” for how much they expedited the task of applying pesticides and plant nutrients to a pecan canopy, the Western Irrigated pecan industry would suffer serious reductions in tree growth and yield from zinc deficiency rosette disorder (even though some progress has been made with soil-applied zinc chelates). The southeastern pecan industry would suffer serious crop loss and decline in tree health from pecan scab disease and other assorted disorders (even though some progress has been made with scab-resistant pecan varieties). You could also expect pecan nut casebearer, pecan weevil, hickory shuckworm, stinkbugs, and other leaf and nut-attacking insects to decimate the U.S. pecan crop annually if growers were forced to rely solely upon hand-held sprayers or airplanes to apply insecticides. Airplanes can help somewhat, but they simply don’t apply enough material to achieve the vertical penetration needed to adequately control those direct, nut-attacking pests throughout the canopy (Bock and Hotchkiss, 2020). Perhaps a helicopter could do a good job, but then pecan farming would be restricted to those men and women who could afford a helicopter pilot on their staff or fly one themselves. So, I rest my case; an airblast sprayer is the most important equipment every grower owns.

From my 32+ years of experience working with pecan growers in Alabama, Texas, and elsewhere, I find tree spraying to be a vulnerability for the pecan industry. Many growers forfeit the cost-benefit of their most important equipment asset by not being well educated on the engineering theory that underpins airblast sprayer technology. Many also do not fully understand pesticide and fertilizer application rates or how to properly calibrate a sprayer. Others cut corners on sprayer purchase price or maintenance, attempting to spray too many trees or trees too large for the size, age, or mechanical condition of the sprayer that they presently own. I attest that sprayer performance matters greatly to future pecan orchard yields and the quality of pecans grown across the U.S. pecan industry.

Because zinc and fungicides need to be applied to new, developing leaves, budbreak and tree spraying coincide. With the 2021 pecan crop starting then as early as mid-March for some growers, I want to provide some background education on the use and operation of airblast sprayers that could help some growers achieve better results against the insect, disease, and nutritional challenges that will surely manifest this year. Here are four statements that will hopefully enlighten some pecan growers on how to get better performance from their most important orchard equipment asset.

We’re not actually “spraying” the trees.

Yes, I’m going against the grain here to attempt to change a fixture in pecan orchard management language, but here it goes. A better term for “spraying” trees with an airblast “sprayer” would be “disrupting” trees with an airblast “infuser.” The compound word “air-blast” hits accurately at the concept I want to describe by suggesting that the physics behind this process is to use a blast of air to displace the resident air in a tree canopy that is harboring insects and supporting disease development with air infused with a pesticide or fertilizer. When canopy air is exchanged with chemical-infused air, the pesticide or fertilizer can be much better dispersed and distributed than if we attempted to spray droplets to all leaf and shuck surfaces scattered within and around a pecan tree canopy.

Mature pecan tree canopies are so dense, layered, and complex that pesticide deposition on all surfaces is nigh unto impossible without the aid of an airblast sprayer fan to exchange the air within the canopy very quickly and infuse it with pesticides and fertilizers. This principle for the grower implies that the air direction from the sprayer volute into and through the canopy is very important.

Sprayers use “volutes,” a housing around the nozzles that channels air into a defined direction. Some designs have adjustable volutes that can rotate/adjust during operation. Others use a fixed pecan volute that channels 180 degrees of air movement into 90+/- degrees with deflectors or gates included to orient some of the air more vertical to reach higher points in the canopy. The orchard manager must adjust the volute where possible to get the best “disrupting” of the central canopy and thus optimal coverage. Managers must also adjust their drive-path in order to move a significant volume of chemical-infused air through the center of the canopy from low to high, and therefore, to achieve the desired air displacement effect. A commonly recommended practice is to configure the sprayer with volute and nozzle location so that 65 to 75 percent of the infused air is directed to the upper half of the tree canopy, and 25 to 35 percent is directed to the lower tree canopy.

Low-hanging tree limbs can be a problem for canopy air disruption and infusion (or “spraying”). Many enclosed cab tractors used in the industry are built for field crops. They can be too tall and vertical, forcing the spray equipment out wide of the tree canopy to avoid broken windshields. The problem is aggravated in orchards that intentionally leave limbs hanging low to gain production.

A detailed study of fruit location on a 26-year-old ‘Success’ pecan tree showed that fruit distribution as a function of height was normally distributed (followed a bell-shaped curve) and that branches lower than 12 feet accounted for only 13 percent of the total nut crop, while two-thirds of the crop was found at heights from 12 to 25 feet. (Lozano-Gonzales, et al., 1992). Low-hanging limbs deflect and thwart too much air for the nuts that they can bear, causing potential for sub-optimal insect and disease control or zinc uptake. If tree limbs in your orchard are forcing your drive path to be completely exterior to the tree canopy, I believe you are doing more spraying at the canopy rather than causing internal canopy disruption, which may negatively impact overall results.

Orchard hedging, a standard practice in the irrigated western region of the Pecan Belt, is ideal for pesticide disruption and infusion with airblast equipment. A more compact and uniform canopy has less wind deflection from low-hanging limbs, and thus, better air infusion and droplet deposition throughout the canopy. The southeastern pecan-growing region in the U.S. could improve scab management by utilizing hedge pruning to enhance spray coverage (Wells, 2020).

Time is of the essence.

Good canopy air disruption and pesticide infusion are more difficult to achieve at faster ground travel speeds. The goal of airblast disrupting and infusing is to get droplets into the canopy. The slower the ground speed, the more spray drops penetrate the canopy (Deveau, 2015). Tractors pulling airblast sprayers faster than three miles per hour, in my opinion, are sacrificing important scab control, PNC control, stinkbug control, zinc uptake, etc., in order to get the job done quicker.

The problem is tree numbers and the cost to obtain additional sprayers. For every one thousand pecan trees spaced thirty-five feet apart (28.6 acres), it takes 5.3 hours to spray both sides of every tree if the tractor throttle is set at 2.5 mph. That’s if every tree is in one single row. This estimate does not take into account traveling between rows (and most orchards are laid out with multiple rows), end-of-row turning, or distance from the fill-up station to the orchard. Airblasting two hundred acres of pecan trees, therefore, is at least a 40-hour task. Growers must carefully assess their time requirement for spraying. Where adding a second airblast sprayer is not feasible, they must be proactive in scouting and monitoring weather to avoid getting behind in a pest outbreak or major scab infection, which then forces them to rush through an insecticide or fungicide application and get less than optimal results.

Calibration should not be guessed, estimated, or “eye-balled.”

Perhaps, university researchers and Extension personnel like myself made the subject of sprayer calibration boring, unappealing, or hard to understand. I’ve met many growers who either really didn’t have an exact process for calibrating their most important piece of orchard equipment or didn’t re-calibrate it very often (or both). Calibration involves three main steps: 1) Equipment configuration for optimal coverage; 2) Mathematical computation of gallons per acre delivery (GPA); and 3) Adjusting equipment to desired levels.

Configuration, the first step, is a qualitative assessment of whether the airblast sprayer, pulled at the desired ground travel speed and with X nozzles of a certain orifice size, switched on or off, will provide the desired canopy air displacement and leaf/nut tissue coverage. Here is where many growers in the industry stumble. Pecan trees are tall. No other orchard crops get as tall as pecan trees (Bock and Hotchkiss, 2020), making mid and upper canopy evaluation of mature trees challenging.

A lot of growers don’t own lift equipment to get up in the canopy and examine whether leaves or nuts at certain locations are getting coverage. I argue that this step is critical to the short-term and long-term financial success of a pecan orchard and justifies renting lift equipment if needed.

With access to lift equipment, orchard operators can use water-sensitive cards pinned or clipped at strategic locations in the canopy to evaluate how well droplets are moving in and around nuts and leaves above the low canopy levels. The spray sensitive cards may be attached to ropes on pulleys or affixed to telescoping poles to allow more than visual assessment from the ground. Another option is to spray kaolinite clay (Surround WP), a white chalky material that will leave telltale evidence of your coverage. Surround is not easy to spray, needs a high degree of in-tank agitation, and is prone to stop up filters, but the hassle is justified if it leads to better canopy disruption and infusion.

Visual assessment of the airblasting process is invaluable to improving airblast operation. Operators often don’t see the canopy penetration and coverage from the seat of the tractor cab. It can be helpful for the operator to watch someone else operate the equipment or watch a video of themselves operating the equipment in order to visualize where improvements can be made.. It’s not unlike use of video replay in sporting events. Video assessment from different angles can identify weaknesses in canopy penetration that result from excessive speed, travel path, etc.

The second step in calibration requires the actual mathematical computation of the solution output of gallons per acre. This mathematical computation of GPA is easily derived with the following formula:

GPA=(Gallons Per Minute) x 1000/(divided by) (miles per hour travel speed) x (feet of row spacing)

GPA=(Gallons Per Minute) delivery of all spray nozzles together) x 1000/(divided by) (miles per hour travel speed) x (feet of row spacing)

By knowing the actual tractor ground speed that gave good coverage in the configuration step (and as discussed above is less than 3.0 mph) and determining the output of the nozzles by performing a test run at operating pressure to determine how many minutes it takes to spray X number gallons, it is easy to calculate an example like the following:

GPA=8.95 gallons GPM x 1000/(2.7 mph) x 35 feet spacing=94.7 GPA

Now, knowing GPA is 94.7 means that the operator is treating 5.3 acres with a 500-gallon spray tank and should add insecticides and fungicides according to the product rate x 5.3 acres.

Growers who get the best results are those that are willing to go back after a calibration calculation and make adjustments (increasing or decreasing MPH or GPM) to increase or decrease GPA and alter the potential product rates of delivery on their trees.

A little preventative maintenance is worth many pounds of pecans.

A pecan grower in Central Texas recently told me that he had bought a new sprayer after a break-down in June last year caused him to miss key sprays for scab control. Sprayers endure incredible wear and tear each season. Zinc, one of the most sprayed products in the pecan industry, corrodes and erodes nozzle tips, filters, and other moving parts. Belts break. Bearings in pumps go bad. Filter screens stop up. Tanks corrode and leak. Airblast sprayers are expensive to replace, so they get pushed to the brink of dysfunction in many orchards.

Because of airblast sprayers’ importance to successful pecan production and quality, I believe every commercial orchard should embrace a culture of maintenance and replacement. Each crop year, sprayers should get a thorough, exhaustive maintenance work-over that anticipates and limits potential problems. Don’t skimp on sprayer maintenance! With no backup sprayer in place, an in-season breakdown could be extremely costly—possibly impacting this year and next year’s crop.

The subject of airblast disrupting and infusing—(Ok, keep calling it spraying if you want to…)—is deeper and more technical than what these four simple statements cover. Wind speed in the orchard during airblasting is important. Nozzle orifice diameter and whirl patterns are important. Mixing order of pesticides is important. To use or not use spray adjuvants could be an entire article. Should you spray when heavy dew is present? Perhaps we need to continue this subject with additional articles? For those of you who want to invest more effort in improving airblast sprayer performance, I recommend Dr. Jason Deveau’s Airblast 101 Handbook (See References). It’s a 200-page read on the subject, but I believe those who review it will be provoked to look carefully at calibration, operation, and maintenance of their airblast equipment.

Bock, C.H. and M. Hotchkiss. 2020. A comparison of ground-based air-blast sprayer and aircraft application of fungicides to manage Scab in tall pecan trees. Plant Disease 104(6),
Deveau, J. 2015. Airblast 101: A handbook of best practices in airblast spraying. Ontario Ministry of Agriculture, Food and Rural Affairs.
Lozano-Gonzales, R., J.B. Storey and M.K. Harris. 1992. Three-dimensional characterization of bearing pecan tree. HortScience 27(11): 1181-1183.
Wells, L. 2020. Make the most of hedge pruning in the Southeast. Pecan South 53(10).
Author Photo

Monte Nesbitt

Nesbitt is an Extension Horticulturist—Pecans, Texas A&M AgriLife Extension, College Station. [email protected]