Assessing Airblast Coverage

Posted on

About Jason Deveau (Spray Guy)

Dr. Jason Deveau has been the OMAFRA Application Technology Specialist since '08. He researches and teaches methods to improve the safe, effective and efficient application of agricultural sprays in specialty crops, field crops and controlled environments. He is the co-administrator of Sprayers101, co-author of the Airblast101 Textbook, a slow cyclist and an even slower runner.

See all posts by Jason Deveau (Spray Guy).

This article describes a method for assessing airblast sprayer coverage based on a protocol developed by Dr. David Manktelow (Applied Research and Technologies Ltd., NZ). It was co-written with David Manktelow and Mark Ledebuhr (Application Insight, LLC, USA).

Why assess coverage?

Airblast sprayer configuration can require a lot of guesswork. There’s too much time between spraying and observing the results for operators to evaluate adjustments. They need timely feedback to assess the fit between sprayer and target.

Achieving adequate coverage of target surfaces (e.g. fruit, wood, inner canopy, etc.) is the basis of effective crop protection. Assessing coverage as soon as possible alerts the operator to correctable problems. The following three methods are helpful, but they have limitations.

The shoulder-check

Shoulder-checks can identify leaks and plugs, but operators cannot detect variation in flow from nozzle-to-nozzle (even when it’s over 50%). The vantage point also makes it difficult to determine if spray is passing under, over or through the target row. It is better to perform an inspection with the help of a partner outside the sprayer as part of a formal pre-spray check.

Shoulder-checks are fine for detecting plugged or leaking nozzles, but the vantage point makes it hard to discern much else.


Unless specified by the pesticide label (e.g. drenching bark with oil), spraying to run-off usually means excessive and non-uniform coverage. It’s been demonstrated in trees and vines that when the outer canopy begins to drip, the inner has only received about half the spray volume.

Efficacy trials confirm that spraying to run-off will provide protection, but it’s unnecessarily wasteful. Loses of 10-15% have been measured at the point run-off. Additional spray volume may increase coverage in the inner canopy, but the saturated outer canopy receives no additional deposit. The excess simply drips off. Further, there are potential phytotoxicity issues at the drip-points where residues concentrate as they dry.

Dripping is an unreliable coverage criterion because the threshold for run-off depends on the nature of the target surface (e.g. waxy, hairy, vertical) and the product formulation (e.g. oils, stickers, spreaders) and droplet size.

Run-off is not a reliable indication of good coverage.

Inspecting wetting and residue

Inspecting targets for wetting or residue can give a broad indication of whether a target received spray, but it’s hard to see on some plant surfaces. Dry fluorescent tracers and kaolin clay can help operators visualize deposits on actual plant surfaces, but they are messy, time-consuming and after-the-fact (i.e. too late too correct sprayer settings).

Helpful Tip: Look for residue on high-contrast infrastructure, like trellises or black irrigation lines. Remember, they are still only subjective indicators.

The preferred method

Water sensitive paper

This method relies on water sensitive papers to help visualize spray coverage. The yellow side can resolve droplets >50µm in diameter, turning blue where it contacts moisture. With the aid of smartphone apps such as SnapCard, or portable scanners such as DropScope, water sensitive papers can be used to characterize droplet density and droplet size up to 30% total coverage.

These surrogate surfaces do not show the spreading effects that can occur on plant surfaces (especially where surfactants are used). They also show lower deposits than leaves, which move freely in airblast air. Nevertheless, they give a useful indication of potential coverage.

To avoid fingerprints, wear gloves or handle them by the back and edges. They will slowly turn blue in humid conditions, so keep them sealed in their foil package when not in use. Packages of 50, 25x75mm (1×3 inch) papers are available online or from local agrichemical retailers for about $50.00.

Preparing water sensitive papers

You will need ten pushpins (five dark coloured, five light coloured), ten papers and a resealable plastic bag. The following process may seem like close-up magic but with practice you can quickly prepare multiple sets of papers.

  1. Remove eleven papers from the foil package.
  2. Stack them yellow-side-up and flip the top paper over.
  3. Using the flipped paper, carefully fold the stack in half. Now ten are folded yellow-side-out.
  4. Expose 1/4 of the middle paper. Pinching the stack firmly will flex it and give support as you pierce the corner with a pushpin (twist as you push).
  5. Use the pushpin as a handle to slide it from the stack and drop it into a Ziploc bag. Repeat the process for the remaining nine papers (return the outside one to the package for later use).
  6. They will remain viable in the sealed bag for several days before they are used in the target canopy. Once placed in the canopy, ten folded papers provide 20 target surfaces.
Six steps to preparing and attaching water sensitive paper.

Placing the papers

To avoid boundary effects, don’t place papers in the periphery of the planting. To make sure the sprayer is up-to-speed and the canopy is not overly exposed to wind, go a short distance into the target row and pick a representative tree or vine. The pins will hold the papers to stems, twigs and leaf petioles. Shadowing from leaves is inevitable, but try to avoid placing them up against fruit, leaves or wood. Relative placement within the canopy depends on canopy size:

Small canopies

For grape, pin five dark pins into stems next to inner bunches deep in canopy. Pin the five light pins around the outer bunches, oriented with one side of the paper exposed to the sprayer. When you stand back, it should be hard to see the inner papers.

Similar positioning can be used for berry canes and bushes. Pin five dark pins in the inner canopy, spanning the height but oriented randomly. Pin five light pins in the outer canopy, spanning the height but oriented with one side exposed to the sprayer.

Helpful Tip: Keep the papers clustered in a 1-2m (6 ft) row and tuck the Ziploc bag into the dripline to mark where they are. They shouldn’t take long to find after spraying.

Medium canopies

For high-density orchards and larger trellised canopies, a ladder might be required. Pin five dark pins in the inner canopy, spanning the height but oriented randomly. Pin five light pins in the outer canopy, spanning the height but oriented randomly

Large canopies

For large trees (e.g. tree nut, citrus, sour cherry), a modified approach is required. Instead of dark pins, use sections of plastic or galvanized conduit. Note the wire clips developed to affix papers to the conduit in the image below. Any method of firmly affixing the papers is acceptable.

Stand at the trunk and raise the conduit section by section to reach the full height of the canopy. Attach five papers as you erect the conduit mast with one at the top, one at the bottom and the other three evenly distributed. A wrap of electrical tape may be required to help hold sections together. Then, pin five light pins in the outer canopy, oriented randomly and spanning as high as can be conveniently reached.

Helpful Tip: Tie a length of flagging tape near the papers to make them easier to find and replace between assessments.

Use lengths of metal or plastic conduit to create a mast that can span the height of large canopies. Distribute papers evenly along the length as it is assembled.
Larger trees, like nuts, citrus and sour cherry make it harder to assess all canopy positions (and to spray them).


It is preferable to spray clean water from a rinsed sprayer, but you can assess coverage during a chemical spray if label restrictions permit re-entry. Always wear PPE when required.

Spray the target row as you normally would (e.g. both sides, alternate row middle, multiple rows) in weather you would normally spray in. Retrieve the papers as soon as they are dry enough to handle.

Assessing canopy coverage

Coverage assessment form

Download a copy of the Canopy Coverage Assessment form.

Complete the top section, being sure to describe the sprayer set-up, application volume and weather conditions at the time of spraying. Either staple or glue the recovered papers to the form. Try to arrange them relative to their original positions in the canopy.

Helpful Tip: Glue sticks work very well but avoid lumps that will show through.

What do you think of the coverage seen on the 20 surfaces in this example?  Seven of the 20 surfaces (35%) show almost no deposit, and 10 (50%) have visibly low numbers of relatively large droplets – that is usually an indication of inadequate coverage.

Assessing each paper

Research and experience suggest that a droplet density of about 85 Fine/Medium-sized droplets per cm2 and about 15% overall coverage is adequate for most foliar insecticides and fungicides. With experience, this can be judged by eye.

Helpful Tip: It is sometimes helpful consider the amount of yellow left between the blue.

Grade each of the 20 surfaces as (E)xcessive, (A)dequate or (I)nadequate by circling the corresponding letter on the form.

  • Adequate satisfies minimal coverage.
  • Excessive will provide crop protection, but often indicates unnecessary waste.
  • Inadequate includes non-uniform coverage and nil coverage.

There are a few notable exceptions:

  • Make allowances for papers where potentially Adequate coverage has been masked by an adjacent obstacle (see paper number 5, below).
  • Finer sprays will have very high droplet counts and less volume. Paper number 6 (below) would be Inadequate for a high volume, dilute application. However, this uniform distribution is Adequate for a low volume, concentrated application (e.g. mistblower).
  • Coarser sprays may have lower droplet counts or coalesce into blobs (see paper number 2, below). Focus on even distribution and the 15% overall coverage.
Unless using a scanner, visually assessing papers can be subjective. Rely on droplet density, overall coverage and the product’s mode of action when making a determination. Paper number 6 is Adequate for a mistblower due to the high droplet density and uniformity.

Assessing the canopy

Spray coverage can be highly variable. This method employs 20 surfaces and semi-random orientation to offset some of that variability. Minimal coverage (i.e. Adequate and Excessive) should be achieved on 80% of the papers.

Complete canopy coverage is not required. Studies in New Zealand winegrapes showed a direct correlation between the percentage of Inadequate papers and levels of bunch botrytis. Disease levels increased as the number of Inadequate papers increased over 20%.

Watch for the following in the overall coverage patterns:

Clustered gaps in coverage

This occurs when spray fails to reach the targets. Gaps often occur in the top third of large canopies and deep in dense canopies. This could indicate problems with air speed/orientation, dense canopies, or inadequate flow from corresponding nozzle positions.

Uneven coverage

In medium and large canopies, the outer canopy often receives more spray than the inner canopy, and this may be unavoidable. Be aware that an even distribution of droplets on poorly covered surfaces could indicate underdosing relative to bluer surfaces.


This is typical in the outer portion of large and/or dense canopies during high volume (i.e. dilute) applications. More than 50% of surfaces will be thoroughly wetted; Papers will curl, and blue dye may drip off. Unless specified on the product label, it is excessive for foliar applications, but may be unavoidable.

In low volume (concentrate) applications, run-off could indicate poor nozzle distribution or tight alleys. It is generally undesirable and indicates waste.

Improving canopy coverage

This is an iterative process requiring a few attempts before coverage is improved. Try to identify the most limiting factor, make a single adjustment, and then reassess. Consider factors such as travel speed, sprayer air output, nozzle rates and overall spray volume. Also consider canopy management and weather conditions.

When water sensitive papers are prepared in advance, each assessment should take two people about 20 minutes. Compare assessments side-by-side. When one set of papers appears “bluer” than another, measurements have shown it represents >20% difference in actual canopy deposits. This is very likely to have a biological impact.

This small investment of time and money can return better crop protection, greater efficiency, and confidence that the airblast sprayer is doing the job.

Real world example

While in Mildura, AU, we were invited to optimize a Silvan wrap-around multirow sprayer in box-hedged grape. Originally an air-shear sprayer, it was converted to employ air induction hollow cone nozzles (six air outlets per row-side, 550 L/ha [50 gpa], 8.5 km/h [5.3 mph]).

A Silvan wrap-around multirow sprayer in grape.

We noted that the outer arms were 2.8m (9 ft) from the canopy, and the inner arms were 2.1m (7 ft). We brought them in to 2.1m and employed the previously described assessment method to establish a baseline for comparison (see assessment number 1).

The outer arm was brought in until nozzles on both arms were equidistant from the target canopy.
Assessment Number 1 (Left). Mostly Inadequate.

Watching as the sprayer passed, we noted the canopy compressed rather than ruffled. This was likely caused by the air outlets being perpendicular to the canopy. When the canopy closed, air and spray were deflected rather than allowed to penetrate. There were also V-shaped plates in each air outlet left over from its days as an air-shear sprayer that deflected the air in strange ways.

We angled the air ducts and decided to remove the 80 degree air induction nozzles. There have been recent reports of improved grape canopy penetration from the new Arag 40 degree hollow cones, so we tried them. Unfortunately, we chose nozzles with too high an output and the operating pressure dropped below 3 bar (44 psi). With poor atomization, the resulting coverage was still poor. Note the finer droplet size from the switch from air induction to conventional hollow cones (see assessment number 2).

Assessment Number 2 (Middle). Finer droplets and still mostly Inadequate.

Time was limited, so we made two significant changes before the final assessment (yes, we know we said one at a time). First, we rearranged the nozzles. 60% of the total volume was from 40 degree nozzles aiming finer droplets at the fruit zone. The remaining 40% was from 80 degree A.I. nozzles aiming coarser, drift-resistant droplets at the upper canopy. This also restored our working pressure.

Then we noticed the position of the nozzles relative to the air outlets. The air preceded the nozzles, which would leave the droplets trailing behind the air rather than carried into the canopy. We turned the nozzle/duct assembly 180 degrees, so the nozzles preceded the air outlet. The final assessment showed greatly improved coverage (see assessment number 3).

Flipped nozzle/air duct assemblies so nozzles preceded air to better entrain the spray.
Assessment Number 3 (Right). Far better coverage, but still room for improvement at the top of the canopy.

The sprayer operator reaped immediate benefit from the two hours of assessment and reconfiguration and has continued to use this method to optimize the match between his sprayers and crops throughout the season.