Category: Coverage

Articles related to horizontal boom sprayer coverage

Coverage is King
We’ve often heard the adage “Coverage is King” but what does that mean, exactly? It means that in order for your spray application to yield acceptable results, a threshold amount of the active ingredient in your tank must end up on the target. But at what point have we achieved sufficient spray coverage without wastefully over-applying to the target? What does good coverage look like?
Let’s manage expectations right here at the beginning of the article: There is no single, definitive answer because it depends on the nature of the application. In other words, you have to understand which factors are relevant to your specific situation before you can understand what success looks like.
Let’s highlight some of those factors:
Transfer Efficiency, Catch Efficiency and Retention
This relates to the spray’s ability to span the distance from nozzle to target (transfer efficiency) get intercepted by that target (catch efficiency) and then deposit a biologically-active residue on the target surface (retention).
- First, the spray must reach the the target location. This may be the soil, or it might be the underside of a leaf deep in a plant canopy. The degree of success will depend on the droplet size(s), distance to the target and the environmental conditions.
- Then the droplets have to be retained by the target surface and not bounce or slide off. Difficult-to-wet surfaces such as fruit, stems and waxy vertical leaves may be more easily covered with finer droplets and/or formulations that include activator adjuvants (e.g. surfactants).
- Then the deposit must stay wet long enough to be absorbed by the tissue, or leave a hardy residue on the surface that can withstand weathering (e.g. precipitation, sun, and even bacteria) long enough to encounter the pest. More on this below.
Mode of Action
This relates to where spray must deposit (or relocate to) in order for it accomplish it’s objective. Here are a few examples of how products might work. Read your pesticide label to determine your situation.
- Some products require contact. Insects must touch them, either via a droplet landing on them or as they move through a deposit. Similarly, certain fungicides must contact fungal hyphae on the plant surface. A few products are designed to drench the target, as is the case with oil-based miticides.
- Some insecticides must be ingested. That may be in the form of a surface deposit or in plant material that has absorbed the chemistry. Similarly, some fungicides are absorbed by plant tissue.
- Many herbicides are mobile (i.e. systemic). They may be drawn up through the roots, or enter the cytoplasm via leaves and travel to the growing points on the plants, or move through the xylem. Others are contact, staying relatively close to the original deposit.
The sprayer operator should consider these factors when planning the application and when evaluating the resulting coverage. So how do we visualize coverage? Some operators look for the shine on leaves, or a cloudy residue once the spray has dried. That’s better than nothing, but we recommend water sensitive paper (WSP), which is still the most versatile and economical way to visualize coverage.
WSP can be purchased from most retailers that carry spray equipment. It is available in three sizes, of which the 1” x 3” size is the most common. It can be folded and clipped to a plant surface, or placed on the ground. We’ve written several articles on how to use it (such as here and here and in pretty much a third of the articles on Sprayers101).
There are two metrics that must be evaluated when assessing coverage on water sensitive paper:
- the area of the target that has spray on it, and
- the distribution of the droplets over that area.
Let’s use a metaphor to explain:
The Battleship® / Coverage Metaphor
Imagine the boats in this Battleship® game are the insect pests, and the board they’re on is a leaf. The white pegs represent the spray deposits. In this first image, we see 100% coverage and a very high deposit density. Sure, we got every boat, but this is literal and figurative overkill. There’s no need to completely drench the target in order to control most pests. When you spray a target past the point of run-off, you are not adding more pesticide to the target – you are displacing what was already there. The surface will not exceed the concentration of product you sprayed (with the possible exception of mixes that include certain adjuvants). While additional volume can improve coverage to a point, there is a diminishing return.
Unless the label specifically asks for a drench, this is too much coverage.
In this second image, we’ve covered about 15% of the target area, which is reasonable. However, note the lack of distribution. You can see that we’ve missed quite a bit of the leaf. If our pretend pests are sedentary and if this was a contact product, then we’ve missed. If this was WSP we would advise the sprayer operator to note how much space there is between the deposits. Could a pest such as an insect or small weed easily fit between the deposits?
15% coverage is good, but the distribution is bad.
In this third image, we are still covering about 15% of the target, but now the spray is distributed more evenly. Some of you are likely noticing that we missed a pest. That observation reminds me of one of my favourite exchanges from the movie “Christmas Vacation” where Clark finally got his house illuminated, but his father-in-law only sees the problems: “The little lights aren’t twinkling.” “I see that and thanks for noticing, Ed.”
15% coverage, distributed evenly. Deposits may have some pest activity beyond the edge of the residue (light red circles).
Yes, we still missed a pest, but spraying is playing a game of odds. You want enough spray to increase the odds of controlling a pest, but not so much to waste spray (and money and time). This image represents an ideal coverage situation. If this pest moves, or this pesticide redistributes even a little, it will affect the pest.
Plus, we should not discount the threshold of influence that lies around pesticide residue. Imagine a small circle around each droplet (illustrated here as light red haloes) where active ingredient may redistribute beyond the initial deposit to affect an adjacent pest. Perhaps even more importantly, deposits do not spread on WSP the way they do on actual plant tissue, so WSP always gives an underestimate of the potential coverage.
In this last image, we see that red deposits have been introduced. This represents a disease control program where an earlier (white) application retains some residual activity when next application (red) is applied. The second spray application almost never lands on top of the first, giving much more protection on the target. For those keeners out there, note that we got that last pest!
In the case of many disease management programs, subsequent sprays tend to fill in gaps left by previous sprays. If timing is prompt, residual activity will see you through.
If you Absolutely Need a Number…
So, what if you’ve read all this but still insist on a firm number to define adequate coverage? We’ll reiterate that there’s no universally-accepted threshold of deposit density or area covered. It would be nice if pesticide labels included this information, but they don’t.
We’ll stick out necks out and say that in general practice we see excellent results when we achieve 85 discrete deposits per cm² as well as 10-15% surface coverage on at least 80% of the water sensitive papers in a spray application. If you can manage this, it should give satisfactory results in most situations.
Ontario Agriculture Conference – 2022
For a really in-depth conversation on the topic of coverage, check out our presentation from the 2022 Ontario Ag Conference. We tried to deliver a fun and memorable demo at the end of this presentation to show how different droplet sizes might contribute to coverage. Enjoy.
January 18, 2022
Broadcast Boom Nozzle Spacing
North American built boom sprayers have nozzle spacings of 20” (50 cm in the rest of the world), but other spacings such as 15” (37 cm) and 10” (25 cm) also exist. What are the reasons for these alternative spacings and do they offer any inherent advantages?
Why spacing matters
Nozzles are spaced along a boom to allow their fans (patterns) to overlap sufficiently at the target. In broadcast spraying, a uniform distribution of spray volume gives us the best chance for consistent coverage along the boom. Since flat fan nozzles produce a tapered pattern (i.e. the volume is highest in the centre and diminishes towards the edges), approximately 100% overlap (i.e. 50% from each neighbour) will produce a uniform swath.
Figure 1: Tapered flat fans that require some overlap are the default pattern type for agricultural boom nozzles. This is true of conventional and low-drift styles. Note that the flat fans are turned 15° to prevent the spray patterns from interfering with one another.
The 100% overlap isn’t just for volumetric distribution. Flat fan spray patterns tend to have more and finer droplets in the centre and fewer and coarser droplets at the edges. All droplet sizes contribute to coverage in different ways, so the overlap ensures both number and sizes are evenly distributed along the entire boom.
Figure 2: 30% overlap may achieve volumetric uniformity. But because the centre of the pattern contains the majority of the smaller droplets, low overlap may result in low coverage in the overlap regions, resulting in striping.
Figure 3: Consistent droplet number distribution along the boom requires at minimum 100% overlap (50% from each neighbouring nozzle). This blends those regions of the patterns with high and low droplet densities.
The generic 20” spacing arose from long-held conventions about boom height, fan angle, and travel speed. Specifically, this spacing required a boom height of 20” to obtain good overlap of the once-dominant 80° fan angle. Combined with 0.15 to 0.3 US gallon per minute (gpm) nozzles and travel speeds of 6 to 8 mph, operators were able to apply 5 to 15 US gallons per acre (gpa) volumes. Using nozzles with smaller flow rates would generally result in nozzle blockages.
But what if we want to change any of those variables? How does this affect nozzle spacing? Figuring out the pros and cons of an alternate spacing requires a little math and some contingency management.
Boom Height Math
First the math. If the boom has 20” nozzle spacing and we need 100% overlap, the width of the spray pattern at target height must be two times the nozzle spacing, which is 40″. You must calculate the required fan angle and boom height to achieve this. Most nozzle catalogues have tables to help with this, or you can download a handy spreadsheet to calculate your own scenarios here.
For today’s standard 110° fans, a minimum boom height of 14” is needed to achieve 100% overlap. For 15” spacing, the height is reduced to 11”. For 10” spacing, we drop to a mere 7”. However, consider that most modern suspended booms are not operated at heights less than 24” to allow for sway. At that height, there’s plenty of overlap to go around for 20″ nozzle spacing. For those booms that are able to operate at a consistent height, narrower spacings permit lower heights that will reduce drift potential significantly. Every time we halve boom height, we also halve drift potential.
Figure 4: Using 110° tips with 20″ spacing, the theoretical height at which we achieve 50% overlap is 11″ above target.
By tilting the nozzles forward or backward from the vertical, we can reduce the boom height somewhat further and still get the same overlap. For example, for 20 and 15” spacings, angling nozzles forward or backwards by 30° allows us to drop the boom another 2” closer to the target.
Contingencies
A suspended boom hardly ever stays at a uniform height; It sways up and down with field conditions, topography, etc. This is why many operators set their booms above the minimum height – to prevent striping when the boom sways low. The penalty is that this increases the distance droplets need to travel, increasing drift potential and any turbulent displacement problems arising from the moving boom.
Assuming a 110° flat fan at 24” boom height, each nozzle achieves a theoretical pattern width of about 70”, which is an overlap of 70÷20=3.4-fold or 240% on 20” nozzle spacing. Given a minimally-acceptable overlap of 50% (25% from each neighbouring nozzle), the boom could be as low as 11”. For 15” spacing, the minimum height for 50% overlap is 8”, and for 10” spacing it’s 5”. This means the narrower spray patterns gain 3” to 6” in allowed downward boom movement.
Figure 5: Using 110° tips on 15″ spacing, the height for 50% overlap is 8″ above target.
A second contingency is that spray patterns are rarely the exact value that the nozzle catalogues specify. A so-called 110° nozzle may operate at only 90°, or up to 150°, depending on the nozzle model, the spray pressure, and the tank mix. Learn more here and here. Patterns also don’t continue to grow at their rated fan angle, as droplets slow due to air-resistance and fall more vertically due to gravity. For that reason, a visual check is recommended to ensure the expected overlap is achieved.
Figure 6: Fan angles indicate initial trajectories of droplets at the edge. With distance, gravity pulls these droplets downward, narrowing the pattern width from that achieved theoretically (figure adapted from image in TeeJet catalogue).
A third issue to consider is less related to boom height but nonetheless affects spray distribution. Small droplets move with air currents, and the turbulence created by large, fast sprayers creates enough turbulence to move these droplets significantly. A perfect pattern under static conditions can look quite different at a fast travel speed with a modest side wind. Low booms may help prevent some of this displacement because droplets spend less time in flight, and their average velocity is faster.
Figure 7: Spray deposition onto a 2 mm string to measure deposit uniformity for a fast travel speed and high boom and a slow speed, low boom configuration.
Flow Rate Math
Flow rate requirements per nozzle change whenever we equip a boom at an alternate spacing. The basic formulae are shown below.
Moving from a 20″ to a 15″ spacing would require a nozzle with 0.75 of the flow rate, approximately from a 02 to 015 size, or 03 to a 025 size, or 04 to 03 size, etc.
Pulse Width Modulation
The use of Pulse Width Modulation (PWM) has increased the overlap requirement. With PWM, alternate nozzles are on a 180° timing offset from their neighbours. This means that when running >50% duty cycle, when one nozzle is temporarily off, its neighbours are on. These neighbours’ patterns must now span the gap, and 100% overlap is the absolute minimum to achieve this. PWM users therefore select the wider pattern angles and some opt for >100% overlap.
Figure 8: Pulse Width Modulated booms require 200% overlap so that the entire boom receives proper coverage when the alternate set of nozzles is off. For 110° fans at 20″ spacing, the minimum boom height would be 21″
PWM Considerations
- High flows (greater than 1 US gpm at the nozzle) that are common for fertilizer top-dressing may require higher-flow PWM valves.
- Narrow spacings reduce the individual nozzle flow rates and can therefore support higher application rates before triggering a larger valve requirement.
- PWM valves aren’t cheap and for example 15″ spacing compared to 20″ spacing adds 24 valves on a 120′ boom.
Banding
We noted that 20” nozzle spacing is a standard because it corresponds to what has traditionally been achievable with available boom heights and spray pattern angles. But things can change.
Narrower spacings such as 15” originate with row crops and planter row spacings of 15” or 30”. These spacings exist so the spray pattern can be placed either over the top of a crop row, or in between the rows for banding. Using narrower fan angles and/or lower boom heights, together with “even” (as opposed to “tapered”) fans, banding sprays can be applied over the top of, or between crop rows. Or drop hoses can reach between the rows for top-dressing or directed sprays into the canopy.
Canopy Penetration
With narrower spacing, it can be argued that a greater proportion of the boom length has spray directed directly downward (corresponding to the centre of the pattern). Whether or not this translates into better penetration of a canopy is a fair question. In laboratory trials, use of 10” or 20” spacing did not improve penetration into a broadleaf canopy. But if the lower boom height afforded by the narrower spacing was utilized, some improvements in the deposit of angled sprays onto vertical targets was observed.
Adjusting to Narrower Spacings
As we showed earlier, use of 15” or 10” spacing booms for broadcast sprays requires a smaller nozzle size to achieve the same spray volumes as the 20” spacing. If boom height remains constant, narrower spacings result in greater pattern overlap which provides more latitude for sway. Alternately, lower boom heights can be used.
Using smaller nozzles on narrower spacing presents some challenges. Generally, smaller nozzle size means finer spray quality. If an operator wants to retain the spray quality they had on a 20″ spacing, they may opt to use lower pressure (not advisable for non-PWM systems) or swap to different nozzle design that can produce the desired spray quality at the lower flow rate.
Smaller nozzles are more prone to plugging, so that needs to be managed with filtration, filling practices and water sourcing. Be aware of the the product formulations and their requirements for filter mesh size. Most dry products specify a 50 mesh filter (or coarser). Also, check size options for nozzles. The smallest size for most nozzle models is 015, but certain PWM-specific nozzles are only available in 03 or larger.
The marriage of narrow spacings with individual nozzle shutoff can result in a versatile system capable of producing high resolution banded sprays in narrow seeded crops. For example, consider a boom with a 10” nozzle spacing spacing that matches the seeder row spacing. The operator can shift from 10” to 20” or 30” from the cab if the valve control software allows it. With accurate guidance and good boom levelling, topdressing foliar products (e.g. nutrients, fungicides) can follow the crop row precisely.
Spot Sprays
Spot sprays present a situation where compromises are needed. Some, such as WEEDit, utilize narrower nozzle spacings to allow better treatment resolution and increase product savings. Any one nozzle or sets of adjacent nozzles may be triggered by the sensor. For single nozzle activation, to preserve the value of the better resolution a uniform, narrow band of spray needs to be created. This means a 30° or 40° fan angle from a banding nozzle will be necessary. For example, a 24” boom height will result in a 13” band with a 30° fan, and an 18” band with a 40° fan. In the latter case, the dose would be diluted by 80%, wasting much of the potential savings.
Figure 10: Boom height is critical for banded sprays and for spot sprays. Too wide a pattern on a single nozzle reduces dose, too narrow creates misses.
Frequently, a patch of weeds will trigger several adjacent nozzles. Now these individual bands need to work together to create a uniform swath. This will inevitably require some overlap to avoid gaps, but too much overlap will result in bands where twice the dose will be applied. A tapered fan may suit this situation better. As a result of these varying needs, tolerances for spot spray boom height are even more strict than for broadcast spraying. More thoughts on spot spray nozzle selection are here.
Conclusions
Narrower nozzle spacings on a broadcast boom allow somewhat lower boom heights and these can in turn reduce drift and improve deposition of sprays. Lower flow nozzles will be needed with narrower spacings, requiring management of plugging and potentially a more drift-prone spray quality. The value of narrower spacings depends on the availability of booms that control sway, allowing them to operate at uniform, low heights.
January 12, 2022
Adventures in Lecturing – Turn Off PowerPoint
Harvest is mostly done and growers want to hear what we’ve learned and what’s coming next. Lecture season is upon us once again.
In 2021 we’re still finding our way through virtual conferences and hybrid models, but I like to think we’re slowly returning to the in-person format. Just last week I gave my first in-person talk in 20 months. It felt wonderful after having spoken into a dead-eyed camera for so long. Half-way through my lecture I remembered a lesson I learned a few years back and spontaneously decided to go off-script.
Let me explain.
In 2016 I was invited to present at the 40th annual Tomato Days conference in Southern Ontario. I knew what I wanted to say, but didn’t have a decent slide deck for that particular topic. I’d have to pull one together.
I work hard on my presentations. I employ lots of imagery (I create all my own illustrations). I get persnickety about fonts, white space and slide transitions. I try to tell a story that educates and hopefully, entertains. Prideful? Perhaps. But if you’re willing to sit on a hard chair for an hour, I’m going to do my best to make it worth your while.
I finished the slide deck, drove three hours to the conference, handed my USB data key to the organizers and sat down to wait my turn. It was a clear, bright winter morning and I saw that the pavilion we were in was more-or-less windows and a roof. It was so bright, in fact, that none of the 150 attendees could see the projector screen!
I watched sympathetically as the first speaker spent 30 minutes trying (and failing) to verbally describe his graphs. I cringed as the second speaker pantomimed her illustrations in some kind of brave, interpretive dance. Then it was my turn.
I decided I wasn’t going down that road.
When the moderator brought up my talk, I turned the useless projector off. I asked the squirming and disinterested audience:
Q. “What’s the most terrifying thing you can do to an academician?”
A. “Take their Power Point away.”

For the next 30 minutes we had a discussion about spray coverage. No props. No slides. The audience slowly warmed up to the new format. They shared experiences. They debated. They asked questions. I became more facilitator than speaker.
When our time was up I think everyone was pleased. Sure, I missed a lot of my key points and never really addressed the subjects I thought I would, but who cares? Everyone learned something.
For me, I learned that speakers should abandon the script every now and again. It’s not always ideal since we’re there to teach and structured visuals are often required. But, the next time you’re asked to speak, consider the possibility of using your time to engage your audience and establish a dialogue… not just talk at them until the moderator gives you the 5-minute warning.
I have a colleague who does this masterfully. Whenever he is the last speaker on the agenda, and the previous speakers have discourteously gone over-time and whittled his time in half, he jumps straight to his take-home slide. He leads a quick discussion with the audience and becomes a hero. The moderators are now back on schedule and no one is late for lunch.
Since “Tomato Days”, I now try to do this once a year. I never know when the mood will take me, but when it does I give the audience a choice: They can hear my canned presentation or I can shut it down and we can have a conversation. To date, given the option, every audience has opted to go off script. It’s scary, it’s fun and like I said earlier, everyone learns something.
I challenge you to try it the next time you’re lucky enough to be in front of an audience in person.
November 29, 2021
Controlling Cercospora Leaf Spot in Sugarbeets
Download the 2023 publication from Crop Protection here.
Cercospora leaf spot (CLS), caused by the fungal pathogen Cercospora beticola, is one of the most damaging foliar diseases affecting sugarbeet (Figure 1) (Khan et al. 2008). Growers rely on broad-spectrum contact fungicides because they are less likely to cause fungicide resistance (OMAFRA 2020). However, these fungicides are usually less effective than other fungicides (Trueman & Burlakoti 2014), and require frequent reapplications (Thind & Hollomon 2018) and good coverage to be effective (Prokop & Veverka 2006; Roehrig et al. 2018).
Figure 1. Cercospora leaf spot on sugarbeet.
We evaluated practices intended to improve the efficacy of Manzate® Pro-Stick™ (Mancozeb) by improving deposition and penetration into the sugarbeet canopy. Practices included different nozzle types (Shepard et al. 2006; Dorr et al. 2013), carrier volumes (Armstrong-Cho et al. 2008; Roehrig et al. 2018; Tedford et al. 2018) and the addition of InterLock®. InterLock is a spray adjuvant made with modified vegetable oil (MVO), vegetable oil and a polyoxyethylene sorbitan fatty acid ester emulsifier. It is intended to reduce the number of drift-prone, fine droplets without compromising the volume median diameter (WinField® 2019).
Research
In 2019 and 2020, InterLock and carrier volume were assessed to evaluate effects of:
1. InterLock on Manzate Pro-Stick efficacy at different carrier volumes.
2. InterLock on spray deposition and penetration within the sugarbeet canopy.
Objective 1: InterLock on Manzate Pro-Stick efficacy at different carrier volumes
Four replicated field trials were conducted at two sites, Dealtown (2019) and Ridgetown (2019 and 2020). Treatments were evaluated using four carrier volumes: 115, 235, 350, and 470 L ha^-1 (12, 25, 37, and 50 gpa) and applied on a 14-day schedule.
Results
- Adding InterLock to Manzate Pro-Stick did not reduce disease accumulation over the season (Figure 2a) or improve beet and sugar yield or sugar quality compared to applications of Manzate Pro-Stick alone (data not shown).
- Carrier volume did not affect disease accumulation (Figure 2b).
Figure 2a. Disease accumulation (standardized area under the disease progress stairs; sAUDPS) (±SE) for fungicide treatments applied to sugarbeets in Ridgetown and Dealtown ON 2019, and in Ridgetown 2020. Bars followed by the same letter are not significantly different at p ≤ 0.05, Tukey’s HSD, ns= not significant.
Figure 2b. Disease accumulation (standardized area under the disease progress stairs; sAUDPS) (±SE) for carrier volume applied to sugarbeets in Ridgetown and Dealtown ON 2019, and in Ridgetown 2020. Bars followed by the same letter are not significantly different at p ≤ 0.05, Tukey’s HSD, ns= not significant.
Objective 2: InterLock on spray deposition and penetration within the sugarbeet canopy
Deposition was evaluated using Rhodamine WT dye recovery. The amount of dye recovered for a treatment (µL AI/ g leaf tissue) was used to make assumptions about treatment deposition in the sugarbeet canopy. To assess spray deposition, samples were taken from six canopy locations (Figure 3 and 4).
Figure 3. Leaf sample collection from sugarbeet canopy.
Figure 4. Leaf samples were taken from a) three canopy locations 1= inner, 2= mid, 3= outer from b) two leaf locations each A= tip, B= base.
Three sets of replicated experiments were conducted in Ridgetown (2019 and 2020) to evaluate the effect of InterLock on canopy deposition when 1) mixed with Manzate Pro-Stick, 2) using three different nozzle types, and 3) using three carrier volumes.
In the first study, four programs (Manzate Pro-Stick + InterLock, Manzate Pro-Stick alone, InterLock alone, and water) were evaluated for dye recovery.
Results
- Deposition was improved for the InterLock only treatment compared with water, but when InterLock was applied with Manzate Pro-Stick the deposition was the same as Manzate Pro-Stick applied alone (Figure 5). It is possible that the fungicide formulation or active ingredient had an antagonistic effect with InterLock, though we cannot determine that from this study.
Figure 5. Effect of program on mean Rhodamine WT active ingredient (µL per gram of dry leaf) (±SE) recovered from six locations in a sugarbeet canopy at the 13 (Trial 1) and 16 (Trial 2) leaf stage in Ridgetown, ON 2019. Bars followed by the same letter are not significantly different at p ≤ 0.05, Tukey’s HSD.
In the second study Manzate Pro-Stick + InterLock and Manzate Pro-Stick were applied using three different nozzle types at ~40 psi:
- The Hardi ISO Injet is an air inclusion 110° flat fan that produces a Very Coarse spray quality.
- The TeeJet XR110 is a conventional 110° flat fan that produces a Medium spray quality.
- The TeeJet AI3070 is an air inclusion, dual flat fan (30° and 70° spray angles) that produces a Coarse spray quality.
Results
- Adding InterLock did not affect deposition and did not alter the performance of any nozzle type (data not shown).
- Deposition among nozzles did differ, with the ISO injet nozzle providing improved deposition compared to the XR110 and AI3070 nozzles (Figure 6).
Figure 6. Effect of nozzle type on mean Rhodamine WT active ingredient (µL per gram of dry leaf) (±SE) recovered from six locations in a sugarbeet canopy at the 15 (Trial 3), 18 (Trial 4), and 19-22 (Trial 5) leaf stage in Ridgetown, ON 2019 and 2020. Bars followed by the same letter are not significantly different at p ≤ 0.05, Tukey’s HSD.
In the third study, Manzate Pro-Stick + InterLock and Manzate Pro-Stick were applied using three carrier volumes: 115, 235, and 350 L ha^-1.
Results
- The addition of InterLock had no effect on deposition, regardless of carrier volume (data not shown).
- Deposition increased with increasing carrier volume (Figure 7a). A regression analysis determined a curvilinear relationship between carrier volume and deposition, predicting that deposition would increase with increased carrier volume until a maximum carrier volume was reached (Figure 7b). Many studies indicate that at exceptionally high carrier volumes coverage can be reduced primarily due to run-off.
- Even though increased carrier volume improved fungicide deposition, increased volume did not improve fungicide efficacy for CLS management (Objective 1 efficacy trials).
Figure 7a. Effect of carrier volume on mean Rhodamine WT active ingredient (µL per gram of dry leaf) (±SE) recovered from six locations in a sugarbeet canopy at the 20 and 23 leaf stage in Ridgetown, ON 2020 (Trial 6 & 7). Bars followed by the same letter are not significantly different at p ≤ 0.05, Tukey’s HSD.
Figure 7b. Regression of carrier volume (115, 235, 350 L ha-1) and mean Rhodamine WT active ingredient (±SE) recovered from six locations in a sugarbeet canopy at the 20 and 23 leaf stage in Ridgetown, ON 2020 (Trial 6 & 7). Data analysis was performed on the log normal scale, means and SE presented have not been back-transformed.”
Canopy location was an important factor in all experiments
The least deposition was always found in the outer and inner canopy from the base of the leaf, and in the outer canopy from the tip of the leaf (Figure 4), suggesting that these locations are the most challenging to achieve spray deposition. An example from the nozzle type experiment is shown in Figure 8. One of the proposed benefits of InterLock is for improved spray penetration, but in the current study, InterLock did not improve penetration of Manzate Pro-Stick into any of the harder to reach canopy locations.
Figure 8. Effect of canopy location on mean Rhodamine WT active ingredient (µL per gram of dry leaf) (±SE) recovered from six locations in a sugarbeet canopy treated with InterLock and different nozzle types at the 15-22 leaf stage in Ridgetown, ON, 2019 and 2020 (Trials 3, 4 & 5). Bars followed by the same letter are not significantly different at p ≤ 0.05, Tukey’s HSD.
Conclusion
Adding InterLock to Manzate Pro-Stick did not improve deposition in any field experiment regardless of the nozzle type or carrier volume used. Further, using InterLock with Manzate Pro-Stick did not improve fungicide efficacy for CLS management. However, we cannot determine from this study if InterLock would improve deposition, penetration, or fungicide efficacy using other fungicide products.
Despite findings of improved disease management with the use of larger carrier volume, fungicides are sometimes still applied with smaller carrier volumes of 100 L ha^-1 or less (Armstrong-Cho et al. 2008; Roehrig et al. 2018) to save time and reduce the cost of application. In this experiment, increased carrier volume improved deposition but did not improve fungicide efficacy of Manzate Pro-Stick for CLS management. There is the potential that using increased carrier volume may be more beneficial in years with a greater disease severity, and may thus be worthwhile to growers, as has been observed in previous research on Cercospora leaf spot in Ontario (Tedford et al. 2018).
See the full thesis here.
This research was sponsored from the Canadian Agricultural Partnership, Ontario Agri-Food Innovation Alliance, Ontario Sugarbeet Growers’s Association, and the Michigan Sugar Company.
References
Armstrong-Cho C, Wolf T, Chongo G, Gan Y, Hogg T, Lafond G, Johnson E, and Banniza S. 2008. The effect of carrier volume on Ascochyta blight (Ascochyta rabiei) control in chickpea. Crop Prot. 27: 1020-1030.
Dorr GJ, Hewitt AJ, Adkins SW, Hanan J, Zhang H, and Noller B. 2013. A comparison of initial spray characteristics produced by agricultural nozzles. Crop Prot. 53: 109-117.
Khan J, del Rio LE, Nelson R, Rivera-Varas V, Secor GA, and Khan MFR. 2008. Survival, dispersal, and primary infection site for Cercospora beticola in sugar beet. Plant Dis. 92: 741-745.
Ontario Ministry of Agriculture, Food, and Rural Affairs (OMAFRA). 2020. Vegetable Crop Protection Guide, Pub 838. Sugarbeets. Queen’s Printer for Ontario, Toronto.
Prokop M, and Veverka K. 2006. Influence of droplet spectra on the efficiency of contact fungicides and mixtures of contact and systemic fungicides. Plant Protect. Sci. 42: 26-33.
Roehrig R, Boller W, Forcelini CA, and Chechi A. 2018. Use of surfactant with different volumes of fungicide application in soybean culture. Eng. Agr. Jaboticabal 38: 577-589.
Shepard D, Agnew M, Fidanza M, Kaminski J, and Dant L. 2006. Selecting nozzles for fungicide spray applications. Golf Course Manag. 74: 83-88.
Tedford SL, Burlakoti RR, Schaafsma AW, and Trueman CL. 2018. Optimizing management of Cercospora leaf spot (Cercospora beticola) of sugarbeet in the wake of fungicide resistance. Can. J. Plant Pathol. 41: 35-46.
Thind TS, and Hollomon DW. 2018. Thiocarbamate fungicides: Reliable tools in resistance management and future outlook. Pest Manag. Sci. 74: 1547-1551.
Trueman CL, and Burlakoti RR. 2014. Evaluation of products for management of Cercospora leaf spot in sugarbeet, 2014. Plant Disease Management Reports. 9: FC009.
WinField United. 2019. InterLock. [Internet]. [cited 2019 Feb 25].
September 21, 2021
Comparing Water Sensitive Paper Brands
Introduction
Spray coverage describes the degree of contact between spray droplets and the target surface area. This metric can be used to predict the success of an application. One of the easiest methods for visualizing coverage is to use water sensitive paper (WSP), which is a passive, artificial collector that turns from yellow to blue when contacted by water.
WSP is often used to evaluate iterative changes to a spray program. Placed strategically throughout a target canopy, or directly on the ground, achieving uniform, threshold coverage translates into improved efficacy, reduced waste, reduced off-target contamination and reduced risk of pesticide resistance development. WSP were also used to develop a system that measures the area covered by the effective radial distance in an attempt to relate the area covered by a stain to a larger area where sufficient pesticide activity is taking place.
WSP tends to underestimate the spreading effect that can occur on plant surfaces (especially when surfactants are used), but they are effective as a relative index.
A brief history of WSP
In 1970, a journal article described a new method for sampling and assessing spray droplets. Photographic paper treated with bromoethyl blue created a yellow surface that changed colour when it encountered moisture. The pH-based reaction was fast and irreversible, leaving a distinct blue stain to mark the deposition.
Ciba-Geigy Ltd. made water sensitive paper commercially available in 1985 (later as Novartis in 1996 and as Syngenta since 2000). It is produced in several formats, but aluminum foil packages of 50, 76 x 22 mm (1 x 3 in.) papers are the most popular. Odds are if you’ve ever used water sensitive paper, it originated from Syngenta in Switzerland. In 2023 I noticed that the papers now say “made in Germany.”
Change of manufacturing location?
In recent years, two new options have been made commercially available: Innoquest’s SpotOn Paper (United States) and WSPaper (Brazil). At the time of writing, there has been no impartial comparative evaluation of these three products.
Once dry, the blue stains on WSP are irreversible and papers can be stored for long periods of time. However unstained portions will continue to react to moisture from humidity, dew, or fingerprints, so care must be taken in their handling and storage.
Comparing WSP brands
The three commercially-available brands of WSP were subjected to a series of comparisons. The intention was not to rank these products, but to determine if they performed in a similar fashion and to alert users to any significant differences.
Packaging and Appearance
Each package was donated for the study. The SpotOn (SO) papers had a “sell-by” date of November 2023, the Syngenta (SY) papers (provided via Spraying Systems Co.) were dated February 2021 and the WSPaper (WS) was their newest formulation (white package, not silver), received June 2021. The comparison was performed on July 5, 2021.
WSP packages.
Each product was a foil or plasticized bag of 50, 26 x 76 mm papers. SO and WS had a re-sealing feature similar to that of a sandwich bag. SO also included a package of silica gel desiccant to capture moisture and a pair of plastic forceps to facilitate handling.
Users are encouraged to label papers to ensure they know their relative position and sprayer pass for later analysis. It was possible to write in ink on the faces of the SY and SO papers, but not WS. It was possible to write on the back of all brands.
The three papers were different shades of yellow. Further, in the author’s experience, the colour can be visibly different between batches of the same brand. In the case of larger experiments where more than 50 papers are required, it would be prudent to ensure papers are not only from the same manufacturer, but the same production batch. This would not be an issue when subjectively comparing papers, but when using software that employs colour thresholding to identify deposits, it could create artifacts. Presently, only Syngenta has a batch number (found on a sticker on the back of the bag).
Bleed-through
WSP is often placed in foliar canopies which are subject to dew and transpiration that can cause the papers to react prematurely. This can be particularly limiting when moisture soaks through the backs of papers. Each brand of paper was placed face-up on a drop of water to see if the water would bleed through.
Three brands were placed on a single drop of water. Within five minutes, WSPaper and Syngenta brands wicked the water through, causing a colour reaction. SpotOn did not, although the yellow surface darkened. When a drop of water was applied to the face, the SpotOn paper still produced a blue stain.
WS quickly curled as the water wicked in from the edges. Within five minutes the water soaked through from the back as well. Within five minutes SY also curled, but the colour reaction was entirely due to water soaking through and not wicking along the edges of the paper. SO did not curl and there was no colour reaction save a minor wicking reaction at one edge. It did however produce a dark yellow patch. In order to see if a colour reaction was still possible, a single drop of water was placed on the face and the colour reaction was distinct and instantaneous.
Note: Others have since replicated this experiment and reported that the response depends on the amount of water used and how long you leave it. We repeated our experiment with higher volumes and longer wait times (see image below). Ultimately, no brand of WSP is water proof from the back. Nevertheless, with small volumes of water (such as from dew) the original assessment of each brand is still valid.
A replication of the bleed-through experiment with the same batch of papers was performed with higher water volumes and a longer duration. Eventually, all three brands bled through. (SpotOn left, WSPaper middle, Syngenta right).
Deformation and drying time
Users of water sensitive paper may be familiar with its occasional tendency to curl when one side is sprayed. In extreme cases, this movement could create smears if the paper contacted other wetted surfaces in dense foliage. The degree of curling was significantly different by brand, with SY becoming convex when wet and then flexing back into a concave form once dry. WS deformed as well, but only to a minor degree. SO did not appear to deform at all. Syngenta has noted that the degree to which their papers curl depends on the batch. Their manufacturing process has changed over the years in response to regulatory requirements and minor adjustments are still occasionally made.
Once dry, each brand of WSP tended to curl to different degrees. Syngenta curled the most and SpotOn the least if at all.
There was no appreciable difference in the time it took for any brand to dry. This is based on attempts to smear papers every 30 seconds. All were dry in under five minutes.
Experimental design
While there is considerable variability inherent to spraying, every effort was made to maintain consistent conditions. Papers were sprayed in a closed room with no appreciable air currents (21.5 °C and 64% RH). Papers were paired randomly, side-by-side on a plastic sled. The sled was pulled at 2.5 kmh (~1.5 mph) through the centre of a spray swath produced by a TeeJet XR80015 positioned 50 cm (20 in.) above the targets. The nozzle operated at 2.75 bar (40 psi) to produce ~270 L/ha (~29 gpa) with Fine spray quality. Six passes were made, producing four sprayed papers for each brand.
All papers were dry to the touch after two minutes. They were removed to a cooler, low humidity space and were digitized and analyzed using the SprayX DropScope (v.2.3.0) within an hour of spraying. We noted that while WS and SO fit easily into the DropScope port, the SY papers were sometimes slightly wider and had to be forced. Learn more about how to digitize and analyze WSP in this series of articles.
Screen capture from DropScope’s smartphone app.
The “ground” option was selected, and each brand of paper was processed using its specific spread factor. DropScope has a detection threshold of 35 µm. This is appropriate as the smallest droplet diameter that can be resolved by any brand of WSP is ~30 µm (Syngenta, Innoquest, SprayX – Personal Communication).
Percent surface covered
The average percent surface covered was calculated with standard error of the mean for each paper. WS and SO produced similar values between 30 and 35%. While all three brands exhibited similar variability, SY approached saturation at approximately 80% coverage. Therefore, WSPaper exhibited a slightly higher degree of spread than SpotOn, while the Syngenta paper exhibited a significantly higher degree of spread.
For reference, it can be difficult to determine if a stain represents a single deposit or is the result of multiple overlapping deposits. This becomes a problem when the surface of the WSP exceeds 20% total coverage. Further, it becomes increasingly difficult to distinguish a stain from the background, unstained surface when papers exceed 50% total coverage.
Average percent surface coverage by brand.
DropScope-digitized images of three brands of WSP. The Syngenta and SpotOn papers were sprayed simultaneously while the WSPaper was sprayed in a subsequent pass. WSPaper exhibited a slightly higher degree of spread than SpotOn, while the Syngenta paper exhibited a significantly higher degree of spread.
Deposit density
The average deposit density is a count of discrete objects (i.e. stains) per cm². WS appeared to resolve the highest count, followed by SY and then SO. The process for determining what is a discrete object, and not the result of anomalies such as overlapping deposits, elliptical deposits or imperfections in the paper itself is complicated and computationally heavy. The algorithms employed by DropScope treated each paper consistently. So, while some differences are attributed to variations in spraying, they also reflect the paper’s innate ability to resolve individual deposits.
Average deposit density was highest for WSPaper, then Syngenta, then SpotOn. Variability was similar in all cases.
Droplet diameter
It is not the intent of this article to determine if WSP should be used to extrapolate the original droplet size. The many assumptions and inconsistencies inherent to this process are well known. Nevertheless, some researchers do use WSP in this manner, so a comparison was warranted.
DropScope bins deposit diameters by size to produce histograms of deposit size by count. These stain diameters are used to extrapolate DV_0.1, DV_0.5 (VMD), DV_0.9 and NMD, which describe the population of droplets that produced the stains. DV_0.5 is the Volume Median Diameter, or the droplet diameter where half the volume is composed of finer droplets and the other half by coarser droplets. Number Median Diameter (NMD) is the droplet diameter where half the total droplets are finer, and half the total droplets are coarser.
Each brand of WSP will permit a certain degree of spread when a droplet of water contacts the surface. This spread factor is specific to the brand of paper. Further, the spread factor is not constant for all droplet sizes; Finer droplets will spread less than coarser droplets.
When processing data using DropScope, selecting the appropriate spread factor makes a significant difference to the output. For example, here are the same four SY papers processed using the Syngenta-specific spread factor as well as the spread factors intended for SpotOn and WSPaper.
The same four Syngenta papers were processed by DropScope using the Syngenta-specific spread factor as well as the SpotOn and WSPaper spread factors. The resulting VMD and NMD were very different.
Therefore, each brand of water sensitive paper was analyzed using its brand-specific spread factor (according to DropScope), to produce the following graph.
Three brands of WSP processed by DropScope using their specific spread factors. VMD differed by as much as 30%.
SY produced a VMD higher than that of WS, and both were higher than SO. There was less variability in the NMD, but this was expected given the high droplet count on the finer side of a hydraulic nozzle’s droplet size spectrum.
Conclusion
Water sensitive paper has immeasurable value in agricultural spraying. It is far more important to encourage its use than to quibble over brands. However, when these tools are used for more rigorous evaluations of spray coverage, brand-specific variability must be addressed.
The differences in how each brand responds to moisture (i.e. discolouration and deformation) may factor into which brand is most appropriate for a given situation. Further, there appear to be significant differences in how each brand resolves coverage. Once again, this may be irrelevant for those spray operators who occasionally use WSP to inform their spraying practices, but for consultants and researchers it is suggested that they use a single brand for an experiment, with papers produced in the same batch run. Learn more about methods for digitizing and analyzing WSP in this series of three articles.
Syngenta, Spraying Systems Co., SprayX, WSPaper and Innoquest are gratefully acknowledged for their contribution of materials and time informing this article.
July 7, 2021