Tag: coverage

How to Spray Ginseng

This article was co-written with Dr. Sean Westerveld, Ontario Ginseng and Herb Specialist.

An effective ginseng protection program begins with observing the Integrated Pest Management (IPM) process:

diagnose the problem,
monitor the problem,
control the problem, and
monitor the results.

When spraying is warranted, the operator should understand the basics of application technology. This not only includes the equipment, but the effects of changing spraying parameters (such as pressure or carrier volume), the impact of weather conditions (such as wind and relative humidity) and the product being applied (such as correct timing and safety requirements). The operator should also understand how to properly maintain, calibrate and orient the sprayer according to the nature of the target. Finally, monitoring the results requires the operator to respond to changes in the environment and target during application and to consider these factors when evaluating the outcome.

The ginseng garden

This is a four-year old garden, which represents one of the largest, densest ginseng canopies an applicator can spray. The six-foot wide beds in this particular garden are higher than most beds, making sprayer/tractor clearance an issue. It also means the distance-to-target from boom to canopy is less in the middle of the bed than it is nearer the alleys, making it difficult to ensure consistent coverage. Sprayer operators typically drive in the same direction over each bed, “training” the plants to bend in the same direction each time the tractor passes over the surface. This practice, combined with fenders on the tractor wheels, helps to minimize physical damage as the sprayer passes.

In a four year old ginseng garden. Clearance is an issue. — Clearance is an issue in a four year old ginseng garden.

The sprayer

This custom-built sprayer is a fairly standard design for most ginseng operations: Eight nozzles on each wing and nine on the centre boom. Spacing varies but this sprayer is on 11 inch centres, with the outermost nozzles on five inch centres and aimed outward towards the adjacent beds. Given the limited boom height, all nozzles are aimed back about 45 degrees to increase the distance to target and allow for overlap. The angle is critical to prevent gaps in the spray swath, but given the recommended practice of limited overlap for hollow cone nozzles, the 11 inch spacing may be a little shorter than required.

Custom-made ginseng sprayer. A standard design. — Custom-made ginseng sprayer. A standard design in Ontario.

Spray coverage

There is no hard and fast rule for spraying ginseng. The crop can receive 30 or more applications a year, most of which are fungicide applications. Tip: Monitoring the small plants inside the canopy is a good indicator of overall garden health.

The following lists products available for use in Ontario at the time this article was published. The application target varies for each product, depending on the pest or disease the applicator wishes to control. As such, the application volume should reflect the location of the intended target. For example, a foliar-and-stem application should achieve consistent coverage of all leaf surfaces without incurring run-off. An application intended to reach the crown through the straw will require some run-off down the plant stem and should require a higher volume than a foliar-and-stem application. Many products will become immobilized if they dry onto the straw. Applications are best done to wet straw, followed by irrigation or rainfall to wash the product into the root zone. Applications for diseases like Rhizoctonia generally take place early in the season before the canopy closes, and higher volumes may not be required to achieve root coverage. In order to know how much is required for optimal coverage, read on.

Table 1 – Spray target and relative volume by pest

Pest	Application Target – Specific Product	Garden Age	Relative Volume
Alternaria and/or Botrytis	Foliar and Stem – all products	Seedling – 2nd year	Low
		3rd – 4th year	Moderate
Phytophthora Leaf Blight	Foliar and Stem – most products	Seedling	Low
		2nd-4th year	Moderate
	Foliar – Aiette and Phostrol	All	Low
Phytophthora Root Rot	Root – xylem-mobile root rot products	All	High
	Foliar – Aiette and Phostrol	All	Low
Phytophthora Leaf and Root	Root – xylem-mobile root rot products	All	High
	Foliar – Aiette and Phostrol	All	High
Cylindrocarpon	Root – all products	All	High
Rhizoctonia	Root – most products	All	High
	Root – Quadris	Seedling	High
Pythium	Root – all products	All	High
Aphids	Foliar and Stem/Berries – all products	All	Moderate
Cutworms	Stem – all products	All	Low
Four-Lined Plant Bug	Foliar – all products	All	Moderate
Leafrollers	Foliar and Stem – all products	All	Moderate
Root Lesion Nematodes	Root – all products	All	High

History of the ginseng boom in Ontario

Historically, ginseng sprayer operators used brass hollow cone nozzles to spray ginseng. For reasons that are unclear, many then adopted the Casotti-style sprayer, which used higher volumes and an oscillating nozzle assembly to create a larger swath. This was determined to be overkill for ginseng, and it produced inconsistent coverage.

Many growers (sadly, not all) switched back to horizontal booms and began using the Arag microjet assembly. Drop nozzles (aka drop arms, drop booms, drop legs, etc.) were positioned with disc-core hollow cone nozzles behind the wheels to direct spray into the canopy from below.

Later, we demonstrated that the microjet mixing valve was difficult to set accurately, creating outputs +/- 50% the optimal rate. In response, a new variation on the Arag microjet was introduced, with a more reliable rate adjustment and a lower price tag (they are imported from Italy by a single North American distributor). The drop nozzles are absolutely critical for under canopy coverage, and growers have begun suspending them in each alley – not just behind the sprayer wheels. I predict the future boom arrangement will return to hollow cone nozzles, but in the form of molded poly nozzles with ceramic handling and drop nozzles with full cone disc-core assemblies. Air assist would be even better.

Sprayer settings

Most operators employ a ground speed of about 5 km/h (3.1 mph), operate at about 13.8 bar (200 psi) with nozzles spaced 25-30.5 cm (10-12”) spraying anywhere from 1,000 L/ha (107 gal./ac.) to 1,686 L/ha (180 gal./ac.). The application volume should reflect the stage of crop growth, the age of the garden and the target in question (see Table 1). Applicators should also consider droplet size (Table 2). This is difficult to control given that the majority use Arag microjets with the 1.5 mm orifice disc. In which case, pressure choice will affect median droplet size, with lower pressures increasing median droplet diameter and vice versa.

Table 2 – The Impact of Droplet Size

Droplet Size	Drops per area	Retention	Canopy Penetration	Drift Potential
Fine	High	High	Low	High
Medium	Moderate	Moderate	Moderate	Moderate
Coarse	Low	Low	High	Low

The older style Arag microjets with 1.5 mm diameter discs have highly variable outputs. We developed tables listing their rates with the mixing valve handle set in two positions. They can be found here. We have also developed tables for the newer Arag nozzles for the 1.0, 1.2 and 1.5 mm discs based on 28 cm (11”) spacing. They are listed in Metric and U.S. Imperial.

Download the tables here: Metric (or) U.S. Imperial

Timed output test

Park the clean sprayer and get the pressure up to the desired level. Using a calibration vessel, perform a timed output test to determine each nozzle rate. I prefer the SpotOn SC-4 and a length of 1” braided line to direct the spray into the vessel. You will get wet, so ensure the water is clean and/or wear appropriate PPE.

At 200 psi, we took readings from each microjet and found that while they were more consistent than the older model, there was still a lot of variation from tip to tip. This required us to turn the valve on the nozzle to get a more consistent output, then take another reading, and repeat until we liked what we saw. It became tricky to adjust the rate without reducing the hollow cone pattern to a solid stream because only a slight turn of the nozzle was required. Once we had it, we tightened the lock nut and moved to the next nozzle. Table 3 is a record of the procedure.

While calibrating, we noticed some of the nozzles would suddenly appear plugged, or dense lines could be seen in the spray cone indicating something was wrong. We cleaned them to discover bits of plastic from the poly tank. I asked about strainers, but they are not available for the microjets. I asked about in-line filters, but they aren’t rated for 200 psi. Filling the tank with clean water is very important, but even more so with these nozzles.

Table 3 – Calibrating the new Arag microjets

Nozzle Position	Rates in gpm (bold represents final rate)	Nozzle Position	Rates in gpm (bold represents final rate)
1	0.97, 0.96, 0.93	14	0.77, 0.92
2	1.07, 1.07, 1.26, 0.9	15	0.76, 0.8, 0.95
3	1.1, 1.1, 1.1, 0.93	16	0.97, 0.95
4	0.73, 0.92	17	0.73, 1.0, 1.07, 1.0, 0.98
5	0.92, 0.92	18	0.83, 0.94
6	0.94	19	0.77, 1.0, 0.99, 1.1, 1.24, 10.8, 0.93
7	0.88	20	0.77, 0.88
8	0.92	21	0.71, 0.95
9	0.95	22	0.77, 1.07, 1.04, 1.1, 1.27, 1.0
10	0.90	23	1.06, 0.97
11	0.86	24	0.77, 0.97
12	0.76, 0.83, 1.0, 1.0, 1.2, 0.92	25	0.68, 0.95
13	0.77, 0.92

Average output: 0.93 gpm, standard deviation of 0.03 gpm.

Ground speed

Once the nozzles were adjusted, we filled the tank ½ full and measured out 25 m in the bed. We would normally do 50 m, but the row was too short. The sprayer operator drove the course and we measured the time it took to travel the 25 m distance. Pass one took 18.5 seconds and pass two took 18.3 seconds. That’s an average of 18.4, which we then double so it works in the formula = 36.8 s.

( 50 × 3.6 ) ÷ 36.8 s = 4.9 km/h

Adjusting the drop leg nozzles

This sprayer had drops behind the wheels and two more to hang in the adjacent alleys. This is excellent because research has shown considerably improved coverage with directed spray from drop arms. In my mind, these are not optional – they are mandatory!

We swapped out the hollow cones we found in those positions for full cone disc and core (D5-DC35). Full cones increase the number of droplets that will clear the raised bed and enter the canopy. When adjusting them, be sure to minimize the portion intercepting the bed, while minimizing the spray escaping up through the canopy. It’s a fine line.

Calculating sprayer output

25 microjets at 200 psi = average of 0.93 gpm = 23.25 gpm
8 × D5-DC35 at 200 psi = 1.4 gpm × 8 = 11.2 gpm
That’s ~34.5 gpm for the boom.
Ground speed was 4.9 k/hr or ~ 3mph.

GPA = (GPM × 5,940) ÷ (mph ÷ nozzle spacing in inches)
GPA = (34.5 gpm x 5,940) ÷ (3.0 mph × 11 inches)
GPA = 204,940 ÷ 33
62.1 GPA or about 580 L/ha.

Diagnosing coverage

Water sensitive paper, which turns from yellow to blue when contacted by moisture, was placed in the ginseng canopy. Two sets of papers were set out, with four papers in each set. The canopy was still wet with rain, which made placement difficult as the papers would accidentally contact water on the leaves and change colour prematurely.

Water-sensitive paper wrapped around tubes for panoramic coverage.

Position#1	Clipped face-down on the underside of leaves at the top of the canopy.
Position#2	Clipped face-up on the upper side of leaves in the middle of the canopy.
Position#3	Clipped face-down on the underside of leaves in the middle of the canopy.
Position#4	Wrapped around a plastic tube and threaded over a wire flag, located at the foot of the plant to give panoramic coverage at the root.

The sprayer passed over the canopy spraying water, and papers were carefully retrieved, allowed to dry and scanned.

Generally, there were no “misses” whatsoever. Position 1 showed excellent coverage, with no indication of run-off and a high droplet count with even distribution. This is ideal for foliar applications, and under-leaf coverage is notoriously difficult to achieve. Positions 2 through 4 showed excessive coverage, with the exception of one of the position 3 papers, which was still adequate.

Example of coverage and paper locations in canopy.

Next steps

Ideally, the operator would drop the pressure by 20 psi increments, reducing output until coverage failed. It is important to note that the operating pressure must never approach the lower end of the nozzle’s recommended pressure range, or the spray quality will be compromised and so will coverage.

Once the coverage is considered a failure, the operator would return to the lowest output that did a good job, and the sprayer is calibrated for that crop (at that stage of growth).

Note that the calibration must be performed for each significantly different crop. With the exception of an early-season drench intended to contact the entire root, an emerging one year old garden would need a very different prescription than a four year old garden with a fully-developed canopy. Plus, the weather conditions will affect coverage, so do not calibrate in conditions you would not normally spray in. Hot and dry and windy conditions produce very different coverage compared to cool, humid and still conditions.

Once the operator knows what each garden requires, they will be able to mix their tanks using the same concentration of carrier to formulated product as they normally use, but likely go further on the tank. It will take some practice before the operator knows how much spray mix is required to finish the job.

November 10, 2022

Spray Coverage in Field Tomato

Spraying field tomato is difficult – period.

In Ontario, early variety tomato canopies get very dense in July. The inner canopy is relatively still, humid, cool and a perfect environment for diseases such as late blight. It is challenging to deliver fungicides to the inner canopy and this can lead to inadequate disease control. Matters are slightly improved as the fruit grows and pulls the canopy open, and staked tomatoes might allow for the use of directed sprays, such as drop arms in staked peppers. But, there’s no getting around it – from a droplet’s perspective, it’s tough to get through the outer canopy.

DSCF0002 — Imagine you are a spray droplet trying to get inside this canopy.

Study 1 – Qualitative Observations

In August, 2011 we worked in a market garden operation in Bolton comparing the spray coverage from four different nozzle configurations. We used the growers typical spray parameters: a travel speed of 4.5 km/h (2.8 mph), an operating pressure of about 4 bar (60 psi), a boom height of 45 cm (18 in) above the ground, and a sprayer output of 550 L/ha (~60 gpa). To monitor spray coverage, water sensitive paper was placed face-up in the middle of the tomato canopy. This diagnostic tool turns from yellow to blue when contacted by spray.

Water-sensitive paper at top of tomato canopy - easy to hit. — Water-sensitive paper at top of tomato canopy – easy to hit.

This particular sprayer was equipped with an air assist sleeve that blew a curtain of air into the canopy at about 100 km/h (65 mph) as indicated by an air speed monitor placed at the air outlet. When properly adjusted, air-assist booms have a number of benefits:

They part the outer canopy giving spray access to the inner canopy.
They rustle leaves to expose all surfaces to spray.
They permit the use of smaller droplets, which are more numerous and adhere to vertical surfaces, by entraining them and reducing drift.
They extend the spray window by permitting the applicator to operate in slightly higher ambient wind speeds.

Boom sprayer with air assist sleeve operating.

We sprayed using the four different nozzle configurations, with and without air assist. Our goal was to make qualitative assessments (Good, Moderate, Poor), and here’s what we observed:

Nozzle Type / Sprayer Output	With Air Assist	Without Air Assist
80 degree flat fans /~550 L/ha (60 g/ac)	Good coverage in upper canopy Poor / Moderate canopy penetration Low drift	Good coverage in upper canopy Poor canopy penetration Moderate drift
80 degree air induction flat fans /~550 L/ha (60 g/ac)	Inconsistent upper canopy coverage Poor canopy penetration “No” drift	Inconsistent upper canopy coverage Poor canopy penetration “No”/Low drift
TwinJet dual 80 degree flat fans /~550 L/ha (60 g/ac)	Good coverage in upper canopy Poor / Moderate canopy penetration Moderate Drift	Good coverage in upper canopy Poor canopy penetration Moderate/High drift
Hollow cones /~750 L/ha (80 g/ac)	Good coverage in upper canopy Moderate canopy penetration Low drift	Good coverage in upper canopy Poor canopy penetration Very High drift

The air induction nozzles performed poorly. Their Coarse/Very Coarse droplets impacted on the outer canopy, created run-off and resulted in very little canopy penetration. Medium droplets produced by twin fans and conventional flat fans were both inconsistent with inner-canopy coverage, but some advantage may have been observed with air assist. The TwinJets contributed to higher drift (likely because they were too high off the canopy) but otherwise produced coverage similar to the conventional flat fans. From these observations, the convention that spray shape (e.g. cone, fan, twin) has little or no impact on broadleaf canopy penetration holds true.

Acceptable spray coverage deep in canopy (harder to hit) using hollow cone nozzles. — Acceptable spray coverage deep in canopy (harder to hit) using hollow cone nozzles and air assist.

After inspecting the papers deep in the canopy, we were surprised that air assist did not obviously improve canopy penetration. It did seem to help, but it wasn’t a slam-dunk. This may be because finer droplets (<50µm) are not easily seen on water sensitive paper. It might also be because we did not calibrate the air speed to the canopy: too little air and spray impacts on the outer canopy, while too much air forces leaves out of the way and spray is blown into the ground. It was obvious that drift was greatly reduced, so logically the spray had to have gone somewhere – we can only assume it entered the canopy.

The best results were achieved with hollow cones and air assist. Theoretically, smaller droplets should improve the potential for coverage by sheer number, but they slow quickly and are easily blown off course. Winds were only about 5 km/h (3 mph) during the trials. Had they been higher, the no-air-assist condition would have resulted in poorer canopy coverage. While we feel the air assist improved inner canopy coverage, we attribute much of the performance to the spray volume of 750 L/ha (80 gpa), which was significantly higher than we used with the other nozzles. When we attempted lower volumes using the hollow cones (not shown) the inner canopy coverage was greatly compromised. Higher volumes are a demonstrated means for improving canopy penetration, so this observation is consistent with what was expected.

The 2011 trial suggested that hollow cone tips used with high volume and air assist, improved canopy coverage and penetration. They are, however, very prone to drift and their use is not recommended without an air assist sleeve to counter the spray drift. Spray volumes over 500 L/ha are highly recommended.

Study 2 – Quantitative Observations

In July, 2016 we ran another study in Chatham-Kent. This operation was concerned about spray drift and recently changed from Hardi hollow cones on 25 cm (10″) centres to TeeJet Turbo TwinJets on 50 cm (20″) centres. They wanted to know if they had improved their coverage. We decided to test four nozzles at similar driving speeds and volumes.

Once again, we used water-sensitive paper. This time we placed two pieces back-to-back (face up and face down) about 1/3 down into the canopy. Then we placed two more in the same orientation about 2/3 down into the canopy. We did this for three plants for each pass. The next four images show the visual drift and weather conditions for each nozzle. Note that only one boom section was nozzled (indicated by a white line) in each condition.

Condition 1 – Turbo TwinJet (Coarse Spray Quality)

Condition 2 – Hollow Cones (10″ centres – Fine/Medium Spray Quality)

Condition 3 – XR 110° FlatFan (Fine Spray Quality)

Condition 4 – TeeJet 3070 (Coarse Spray Quality)

It was very humid, making it difficult to place and retrieve the papers without smearing them. This made it tricky to discern differences in coverage, and the blurring prevented us from quantifying droplet density (i.e. number of drops per unit area). Nevertheless, papers were scanned and the percent coverage was calculated using the DepositScan software developed by the USDA’s Dr. Heping Zhu. The average percent-coverage (± S.E. n=3) is shown in the image below.

Coverage on the upward-facing papers in the upper portion of the canopy showed excessive coverage for all nozzles but the 3070. Little or no coverage was detected on the downward-facing cards, but without air-assist or a directed application (e.g. drop arms), this was expected. It’s the deeper canopy that’s of particular interest. The only significant difference may lie in the XR flat fan which showed more coverage on the upward facing papers and some (however little) on the downward facing papers.

This came as something of a surprise given that the XR produced a Fine spray quality and there was no air assist to guide spray into the canopy. I believe the high humidity and low winds played a role in this outcome by reducing evaporation and off-target drift. On a drier, windier day, we likely would not have seen this level of inner canopy coverage for either the XR or the hollow cone. By comparison, the Turbo TwinJet with its Coarse spray quality not only reduces off target drift, but would be more resilient in drier and windier weather and may very well have produced the best coverage by comparison.

Take Home

Drawing from both studies:

Properly calibrated air assist will reduce drift and has promise to improve canopy penetration/coverage.
Spray shape (e.g. twin, hollow cone, flat fan) does not seem to play a role in canopy penetration.
Spray quality larger than Coarse may negatively impact canopy penetration in tomato.
Coarse spray quality is perhaps the most versatile option when volume is sufficient (>500 L/ha).
Fine-Medium spray quality is only a viable option in high humidity and light winds. However, air assist is critical to counter drift, and high spray volumes (>500 L/ha) are still required despite the higher droplet count.
Underleaf coverage is exceedingly difficult to achieve, even with finer spray quality and air assist.

This occurred in Ontario (date and location withheld). The sprayer missed the outer edge of the tomato field during a late blight application. An unintentional field check, and amazing to see the results.

August 4, 2022

Gear up – Throttle down
In 1977, David Shelton and Kenneth Von Bargen (University of Nebraska) published an article called “10-1977 CC279 Gear Up – Throttle Down”. It described the merits of reducing tractor rpm’s for trailed implements that didn’t need 540 rpm to operate. In 2001 (republished in 2009), Robert Grisso (Extension Engineer with Virginia Cooperative Extension) described the same fuel-saving practice. Again, it was noted that many PTO-driven farm implements don’t need full tractor power, so why waste the fuel? He tested shifting to a higher tractor gear and slowing engine speed to maintain the desired ground speed. 700 diesel tractors were tested, and as long as the equipment could operate at a lower PTO speed and the tractor itself didn’t lug (i.e. overload), as much as 40% of the diesel was saved.
How this applies to Airblast
For airblast operators with PTO-driven sprayers and positive-displacement pumps, this has potential for reducing air energy. Gearing up and throttling down (GUTD) sees the operator reducing the PTO speed from 540 rpm to somewhere between 350-375 rpms, which not only saves fuel but more importantly slows the fan speed. This may be an option when air energy from the sprayer, even at higher travel speeds and a low fan gear, still overblows the target canopy.
Some airblast sprayers, like this one, feature fan blades with manually-adjustable pitch to increase or lower air volume and speed. It’s often a pain to try to adjust them, and most operators only try it once.
A good time to try this out is early in the spraying season when (most) canopies are dormant and at their most sparse. For example, when applying dormant sprays in apple orchards, look to see if the wood on the sprayer-side gets wet, but does not creep around the sides. This suggests that the air, and much of it’s droplet payload, are being deflected. When the air speed is slowed, it will become more diffuse and turbulent on target surfaces, and this turbulence helps more droplets deposit in a panoramic fashion within (not past) the target canopy. Look to see if the wood is wet >50% around the circumference of the branches. You’ll get the rest when you spray form the other side.
Limitations
GUTD is not always appropriate. It requires airblast sprayers with PTO-driven positive displacement pumps (e.g. diaphragm). Airblast sprayers with centrifugal pumps would experience a drop in operating pressure and would have to be re-nozzled. Further, the pump must have sufficient surplus capacity to maintain pressure at low rpms.
GUTD is not intended for air-shear sprayers that employ twin-fluid nozzles because dropping air speed below a certain threshold may compromise spray quality; the air needs to be fast enough to create and direct spray droplets
The tractor must have sufficient horsepower (more than 25% in excess of minimally-required capacity) to permit the reduction in engine torque. This is especially important if the operator is on hilly terrain. If the tractor begins to lug (e.g. black smoke, sluggish response, strange sounds) you’ll be in trouble.
Observations
We first experimented with GUTD in 2013. We noticed how much quieter the sprayer was, and the fuel consumption was certainly reduced. One grower-cooperator switched to a GUTD spray strategy mid-way through their dormant oil application in pears. We saw the trees immediately began to drip. Panoramic coverage was improved significantly; once the operator passed down the other side of the target, capillary action and surface tension helped to give near-complete coverage.
However, in one instance, the operator was already applying a low spray volume per hectare using air induction nozzles and their lowest fan gear. By further slowing fan speed using GUTD, coverage at the top of his cherry trees was compromised.
In short, GUTD can work under the right circumstances. If you want to try it, use water-sensitive paper to establish a base-line with your current practice, and then evaluate coverage after you change your sprayer settings.
May 18, 2022
Drop Hoses Improve Coverage in Field Peppers
In early July 2016, a farm supplier contacted us on behalf of a client with a history of disease control issues in his field pepper operation. He wanted us to calibrate their sprayer and diagnose spray coverage to see if there was room for improvement. Improved coverage doesn’t necessarily mean improved efficacy, but generally it’s a reliable indicator. When we arrived at the field the winds were gusting over 15 km/h, which had the potential to create a massive drift issue. We were only spraying water, so it was decided that if we managed decent coverage in those conditions, there would be no need to worry on an acceptable spray day.
Field pepper in Southern Ontario in mid-July
The grower traditionally ran two different settings on his sprayer. They were relatively low volumes for a vegetable operation, but the crop was still small at this stage, so we did not propose raising the volume:
1. TeeJet AITX 11008’s on 50 cm (20″) centres at 11.25 kmh (7 mph) and 3.44 bar (50 psi). That’s 3.35 L/min (0.89 gpm) per nozzle for a total rate of 350 L/ha (37.5 gpa).
2. TeeJet ConeJet TXVK18’s on 50 cm (20″) centres at 7 kmh (4.5 mph) and 3.44 bar (80 psi). That’s 1.6 L/min (0.42 gpm) per nozzle for a total rate of 275 L/ha (29.5 gpa).
To test the coverage with these settings, we folded a piece of water-sensitive paper over a leaf to cover both surfaces, and wrapped one around a hollow tube to mimic a plant stem (see figure). Three plants were papered for each sprayer pass. Papers were collected, digitized and analysed for percent-coverage and droplet density. When diagnosing coverage for a horticultural crop, a distribution of 85 medium deposits/cm² and 10-15% coverage is a reasonable standard for most applications.
Location of water-sensitive papers in situ.
The first condition (the AITX tips) averaged 17% coverage on upper leaf surfaces (37 deposits/cm²). These were coarser droplets at relatively low volume, so it was no surprise that we didn’t achieve 85 deposit/cm² target. When using such large droplets, it is more important to achieve an even distribution and the 10-15% surface coverage (we achieved 17%). There were no deposits on the underside of the leaves (See figure 1), but that was also expected as coarser droplets tend to follow a downward vector that is not conductive to under-leaf coverage.
Figure 1 – Water-sensitive papers from three plants sprayed in Condition 1. Percent coverage and droplet density are calculated for the leaves, and a visual inspection is made of the stems.
The second condition (the ConeJets) provided better coverage. The fine droplets produced covered an average 17.5% coverage with a distribution of 99 deposits/cm² on upper surfaces, and 23% coverage with a distribution of 185 deposits/cm² on lower surfaces. Panoramic stem coverage was improved as well (see figure 2). This is excellent coverage, but the finer droplets were highly prone to drift (see below). With no form of drift control, this set up is undesirable.
Figure 2 – Water-sensitive papers from three plants sprayed in Condition 2. Percent coverage and droplet density are calculated for the leaves, and a visual inspection is made of the stems.
With no form of drift control, the finer droplets produced by hollow cones create unacceptable spray drift, even in moderate wind conditions.
This led us to propose a more directed boom arrangement: We set up a hollow cone over the row (the grower’s original ConeJet) and a drop hose suspended in each alley with two TeeJet XR 8004 flat fans positioned on an angle (i.e. not vertical or horizontal to ground). This gave sufficient height to span the canopy with as little direct waste on the ground as possible. As the crop grows, the nozzles would need to be twisted into a more vertical alignment.
ConeJet TXVK18’s alternating with drop hoses with TeeJet XR 8004’s.
We did not use an air induction fan to avoid the Very Coarse spray quality and we used 80° instead of 110° to ensure the spray did not overshoot or undershoot the plant. Here are the details of the third set up:
3. TeeJet ConeJet TXVK-18’s on 100 cm (40″) centres at 7 kmh (4.5 mph) and 3.44 bar (80 psi). That’s 1.6 L/min (0.42 gpm) per nozzle. Also, two TeeJet XR 8004’s per drop on 100 cm (40″) centres at 7 kmh (4.5 mph) and 3.44 bar (80 psi). That’s ~4.5 L/min (1.2 gpm) per drop hose. Together, set of nozzle for a total rate of 523 L/ha (56 gpa).
This set up raised the volume considerably and aimed spray directly at the sides of the plant. Coverage was excessive and in a few cases exceeded what the diagnostic software could reliably resolve (see figure 3). Since the plants were still small at this stage, it was decided we would let them “grow into the volume” and come back to check coverage once they were at full size.
Figure 3 – Water-sensitive papers from three plants sprayed in Condition 3. Percent coverage and droplet density are calculated for the leaves, and a visual inspection is made of the stems.
When we returned in mid-August the plants had reached full maturity. In this final coverage trial, we added a second water-sensitive paper to each plant to span the height of the crop canopy, which had grown considerably.
The same pepper plants ~5 weeks later had more than doubled in size.
Coverage was reduced compared to how we left things in July, but appeared to be sufficient on key surfaces (see figure 4). The papers showed upper leaf-surface coverage of 63%-to-offscale and deposit distribution of 137 deposits/cm²-to-offscale. Coverage on the lower leaf surfaces was greatly reduced to 4-4.5% and 36-90 deposits/cm². Panoramic stem coverage was present, but minimal. Applying higher volumes would likely have improved matters.
Figure 4 – Water-sensitive papers from three plants sprayed in Condition 3, ~5 weeks later. Percent coverage and deposit density are calculated for the leaves, and a visual inspection is made of the stems.
When asked about the drop hoses, the grower reported “They are a bit of a nuisance because they take extra time to put on, and they get caught in the bush at the back of the field. But if they increase our coverage, then they’re worth the extra effort.”
Final thoughts
Adding drop hoses to a vegetable sprayer may be unconventional, but if fungicide coverage is a concern, and the drops will fit between rows, they might be worth a try. Carefully consider the volumes you use because they should reflect the size of the plant canopy you are trying to protect. Finally, water-sensitive paper provides excellent feedback to help you decide if your field volume, nozzle rates and nozzle positions are providing acceptable coverage.
April 22, 2022
What’s the Cost of Poor Deposit Uniformity?
We’ve heard it often: calibrate your nozzles to be sure your boom output is uniform across its entire width. The downside of poor uniformity is obvious: strips of over- or under-application causing problems with pest control or crop tolerance. A graduated cylinder held for 30 s under each nozzle is the approach of choice. Several electronic versions exist to make the job easier, for example the Spot On.
But there’s more to the story. Nozzle calibration only ensures volumetric uniformity from nozzle to nozzle. It serves to identify worn, plugged, or damaged nozzles, and little else.
After release, the spray is atomized and distributed across a wider area with a properly developed pattern. An operator adjusts boom height or spray pressure to generate proper overlap for a given fan angle at the target height. Unfortunately, the uniformity of this pattern can’t be measured with a graduated cylinder, so we’ve traditionally used a “patternator”, a flat collector placed under a few nozzles that uses a series of channels to show the peaks and valleys of the volumetric distribution. Both calibration and patternation are done with a stationary spray boom. Nozzle manufacturers employ both methods to ensure their products meet international uniformity standards before marketing.
A spray patternator determines the uniformity of a stationary boom’s spray distribution (Photo: TeeJet)
Burt even that isn’t enough. We can have good volumetric distribution but still have inconsistent coverage in places. To identify those regions, we need a way to measure small amounts of spray deposit under a moving boom, ideally in the canopy we intend to treat. Here we have a few options. We can place a tracer (dye, salt, etc.) in the tank, and collect spray on small collectors placed throughout the area to be treated. We collect the samples, wash them, and analyze the solvent for the tracer. This requires special equipment and takes time. It’s useful, but only measures dose, not droplet size or density.
Plastic straws can act as collectors of sprays under field conditions.
Monofilament strings can be used to collect spray over long distances.
A faster way is to use water-sensitive paper, about which we’ve written here and here. Using WSP is fast and easy, and it can provide additional information such as the number of droplets per unit area, or the total percent of the area covered, or even the size of the deposits, with the right equipment. We call this “coverage”, and believe this to be one of the two components of good pest control (the other being “dose”, the total amount of material deposited). Because the world isn’t fair, WSP isn’t great at quantifying dose.
Water-Sensitive paper provides a quick visual indication of the deposit, not just amount but also qualitative aspects such as droplet size and distribution.
The industry has done a good job of identifying the dose required for good control, and this is reflected in the rate recommendations on a label. But there are a few gaps. They don’t tell us, for example, what “good coverage” is, despite often telling us to “ensure” it.
Back to Deposit Uniformity
We quantify deposit uniformity by calculating the Coefficient of Variation (CV) of a series of measurements. The CV is defined as the standard deviation of these measurements, expressed as a percent of the mean value.
Because it’s hard to measure, it’s easy to ignore. But here are a few basics our research has told us: (In the first three examples, deposits were measured under a spray boom using petri plate or drinking straw samplers. There was no interference from a canopy. The last example was taken from within a canopy.)
- When measuring the deposited dose, the CV under a boom tended to rise with increased wind speed. This is no surprise, as it reflects that more wind has a greater chance to displace spray from its intended destination.
Spray deposit uniformity, observed during various spray drift studies, tended to decrease with higher wind speeds.
- Higher booms and increased travel speed also tended to increase deposit CV.
Faster travel speeds during spray drift studies tended to decrease uniformity.
- Finer sprays tended to increase deposit CV. This makes sense, as the finer droplets are more easily displaced by air movement.
Coarser sprays created more uniform deposits possibly because they were more resistant to turbulent displacement.
- Deposits were reduced and became more variable deeper in a broadleaf canopy. Again this makes sense, as there are a lot of obstacles to clear and canopies themselves are by no means uniform.
Deposit amount was lower in the canopy, as expected. But the lower deposit was also more variable.
Also note that the CV in the canopy was quite a bit higher (40 – 60%) than for the exposed targets (10 – 20%). That’s another challenge.
To recap, the best uniformity was achieved with low booms (as long as patterns overlap sufficiently), slow speeds, low winds, and coarser sprays. It’s easy to see that current spray practice isn’t always conducive to uniform deposits.
Deposit variability as captured by a 2 mm diameter string with two sprayer configurations.
So What?
Why does uniformity matter? It matters because more variable deposits are less efficient. They require higher doses for the same effect as uniform deposits. Here’s why:
The figure below shows a typical dose response curve for a herbicide. On the y-axis, we see weed biomass, on the x-axis herbicide dose. At low pesticide doses, not much happens. (In fact, we often see a slight increase in biomass with very low herbicide doses.) As we increase dose, biomass begins to decline, and as dose increases further, the effect begins to taper off. At a certain dose, no further biological response is possible.
A typical dose response curve for a herbicide.
In the next figure, we see that application of a uniform dose “a” results in biomass “y”, about 20% of untreated.
A dose response curve represents the weed biomass that resulted from any applied dose.
Next, we apply the same average dose, but we do it non-uniformly. At some locations under the boom, the deposit may be 40% higher or lower than average. The result is response “z”. Weed control is worse, as bad as it would have been at a lower uniformly applied dose (effective dose “b”).
A variable dose across a field results in many individual weed biomasses because of deposit variation. The net result is lower control.
This effect only happens when the effective dose is near the lower inflection point of the dose response curve. Perhaps we’re shaving rates. Perhaps the weather is challenging the herbicide’s performance. Or perhaps the weed is difficult to control. Under those conditions, any gain in performance with a higher dose is less than the penalty from a lower dose.
There are two ways to correct this performance loss. One is to apply a higher herbicide rate. It’s commonly done, as insurance against – you guessed it – variability, and it’s one reason why label rates have some flexibility. The second way is to improve deposit uniformity. In effect, better uniformity allows for rate reductions.
Label rates are typically in the flat region of the dose response curve to allow for variable conditions in weed susceptibility, weed growth stage, growing conditions, and deposit variability.
Take Home Message
Uniform spray deposition improves overall control. Our examples used herbicides, but the same is true for fungicides and insecticides. It’s true for field crops as well as fruit and vegetable sprays.
Uniformity is especially important when the application is done under adverse conditions in which the pesticide performance is challenged. It’s a fundamental part of good application practice.
It’s not always easy to improve uniformity. But at least it should be measured. Without measuring it, an applicator may never know how much product is being wasted. Have a look at the Crop Adapted Spraying approach Jason is using, it’s a template for all sorts of applications.
What can you do? The easiest task is to record the flow from each nozzle. The results might be surprising. Ensuring proper and consistent boom height is also important. Using water-sensitive paper to visualize the quality of the job would be icing on the cake. And adjusting application method, with uniformity as a goal…that gets you a gold star.
February 3, 2022