Category: Drift

Hort articles about spray drift and mitigation.

Disease Control in Berry Crops

In the spring of 2016, the Ontario Berry Growers Association (OBGA) conducted a survey of its membership to poll how fungicides were being applied. The results were very interesting.

Fungicide basics

Generally, fungicides registered for berry crops are contact products, so coverage and timing are very important. The fungicide has to be distributed evenly on the target before disease has a chance to infect the crop. That means the sprayer operator must be aware of the susceptibility of the crop to the level of disease pressure to ensure timing is appropriate. While kickback and post-application distribution of pesticide residue is sometimes possible, sprayer operators should not rely on it. The following table outlines application recommendations for a fungicide commonly used in Ontario. It combines labelled information and provincial recommendations and is representative of most fungicides.

	Summer-fruiting and Fall-bearing Raspberry / Blackberry	Highbush Blueberry	Day-neutral and June-bearing Strawberry
Labelled rate	2.5 kg/ha	2.25 kg in 1,000 L/ha	2.75-4.25 kg in 1,000 L/ha
Diseases (Labelled and Ontario provincial recommendations)	Anthracnose fruit rot, Spur blight, Leaf spot, Botrytis grey mould	Anthracnose fruit rot, Shoot blight (Mummy berry), Botrytis twig and/or blossom blight	Common leaf spot, Botrytis grey mold
Crop staging	Bloom, Pre-harvest, Harvest	First bloom, Fruit ripening	Flower bud, First bloom, 7-10 days after bloom, Pre-harvest, Through to fall

As of 2016

The spray target

The applicator reading the recommendations should be considering the best way to get the fungicide to the target. But, what is the target, and what is the best way to apply it? It seems the recommendations raise as many questions as they answer:

With the possible exception of blueberry, this fungicide can be applied through much of the growing season (especially when it’s been a wet season). That means the crop staging is highly variable.
The primary target is blossoms, but depending on the disease, leaves and stems are also important.
The label states a volume of carrier (i.e. 1,000 L/ha) for strawberry and blueberry, but not the cane fruit. It does not specify highbush blueberry versus the sessile, ground cover variety.

So, this means is the sprayer operator has to spray crops with highly variable physiology (e.g. bush, cane or sessile row crops), onto very different targets (e.g. leaves, canes, stems, flowers) throughout much of the season as the crop canopies grow and fill. This is a very challenging spray application. It would be wrong to suggest a single spray quality, water volume or sprayer set-up to efficiently accomplish all these goals (more on that later). The first consideration is the application equipment itself.

The application equipment

Berry growers employ a variety of sprayers to protect berries. Without considering models or optional features, there are three fundamentally different styles: Airblast, backpack and boom. According to the survey, the following table shows which sprayers are used in which berry crop in Ontario. Approximately 60 growers responded, and many grow more than one variety of berry and use more than one style of sprayer.

	Airblast Sprayer	Backpack or Wand Sprayer	Vert. or Hor. Boom Sprayer	Total
Highbush blueberry	8	1	0	9
Day-neutral Strawberry	3	0	21	24
June-bearing Strawberry	5	0	32	37
Raspberries & Blackberries	21	1	7	29
Total	37	2	60

So, generally, cane and bush berries are sprayed using airblast sprayers and strawberries using horizontal booms. The survey didn’t specify features such as air-assist on booms, or whether or not those booms are trailed or self-propelled. The type of, and features on, any given sprayer dictate the limits of what an operator can adjust to improve coverage.

Water volume

Respondents also reported on how much carrier (i.e. water) they used to spray fungicide on their crops. Given Canada’s propensity to report volumes in many different forms, I have converted all values into the most common units: L/ha, US g/ac and the dreaded L/ac:

	n	L/ha ± std (max./min.)	US g/ac ± std (max./min.)	L/ac ± std (max./min.)
Highbush Blueberries	7	534.2 ± 340.1 (1,000/150)	57.1 ± 36.4 (106.9/16)	216.2 ± 138 (404.7/60.7)
Day-neutral Strawberries	22	418.5 ± 192.2 (1,000/224.5)	44.7 ± 20.6 (106.9/24)	169.4 ± 77.8 (404.7/90.8)
June-bearing Strawberries	33	403.1 ± 235.1 (1,000/50)	43.1 ± 25.1 (106.9/5.3)	163.1 ± 95.1 (404.7/20.2)
Raspberries & Blackberries	27	450.1 ± 279.4 (1,200/50)	48.1 ± 29.9 (128.3/5.3)	182.1 ± 113.1 (485.6/20.2)

There appears to be a lot of variability in the volumes applied, but on the whole, very few are using the 1,000 l/ha indicated in the fungicide recommendations. The ~430 l/ha overall average is no surprise; labelled volumes are quite often higher than what sprayer operators use. In some cases, high label volumes are warranted because the product requires a “drench” application to totally saturate the target, or to penetrate very dense canopies. Conversely, a high label volume might reflect outdated practices if that label hasn’t kept up with current cropping methods or application technology. Sometimes label volumes are suspiciously large, round numbers that suggest they are intended to encompass a worst-case scenario (e.g. a large, unmanaged crop with high disease pressure and a less-than-accurate spray application). In the particular case of crops sprayed with an airblast sprayer, it is very difficult for a label to accurately predict an appropriate volume due to the variability in crop size, density and plant spacing. This has led to methods to interpret labels, such as crop-adapted spraying.

The disparity between label language and grower practices is not entirely the fault of the label. Most sprayer operators don’t want to carry a lot of water because more refills prolong the spray day. In situations where the crop has reached a critical disease threshold, or bad weather has compressed the spray window, sprayer operators sometimes reduce the volumes in the belief that “getting something on” trumps “good coverage”. Perhaps that’s true, but insufficient volumes greatly reduce coverage. This can be further exacerbated when operators do not account for the increase in crop size and density over the season, or the impact of hot dry weather on droplet evaporation.

Improving coverage

So, is there an ideal sprayer set up and volume? As previously alluded, the variability in crop staging, crop morphology, target location and spray equipment make a single recommendation impossible. But that doesn’t mean there aren’t diagnostic tools and a few simple rules to help a sprayer operator determine a volume to suit their particular needs. Much can be accomplished with these three things:

Water-sensitive paper
A modest selection of nozzles and a nozzle catalogue
An open-minded sprayer operator willing to spend a little time and reconsider traditional practices

Rule-of-thumb fungicide coverage on water-sensitive paper.

Water-sensitive paper is placed in the canopy, oriented to represent the target (e.g. leaf, bloom, etc.). It is important to put multiple papers in at least three plants to ensure the coverage reflects a typical application. The paper changes colour when it’s sprayed and this provides valuable and immediate feedback. Did the spray go where it was supposed to go and did it distribute throughout the target? If so, then the operator now knows that they can safely focus on timing rather than targeting. If not, a little diagnosis is required:

1. Were targets completely drenched? If so, there is too much coverage. Operators can drive faster (if possible, and as long as it doesn’t create drift), reduce operating pressure (if possible, and as long as the nozzle is still operating in the middle of its registered range), or change nozzles to lower rates (as long as spray quality is constant).

2 .Were targets only partially covered, as if a leaf obstructed part of the target and created a shadow? This mutual-shading is the bane of spraying dense canopies. One possible solution lies in understanding droplet behaviour: Coarser sprays generally mean fewer droplets and they move in straight lines. Therefore, when they hit a target, they might splatter or run-off, but typically their journey is over. If the spray is too Coarse, a slightly Finer spray quality increases droplet counts and may help droplets navigate around obstacles and adhere to more surfaces. Sprays that are too Fine will not penetrate dense canopies without some form of air assist. They slow very quickly and tend to drift and evaporate before they get deep enough into a canopy to do any good. A Medium droplet size is a good compromise because it produces some Fines and some Coarser drops – the best of both worlds.

Increasing volumes and reconsidering spray quality often helps, but there might be other options. If using air assist, there are tests that can confirm the air volume and direction are appropriate. Another solution might lie in canopy management (where pruning bushes and canes can help spray penetration immensely). Still another might lie in the use of adjuvants to improve droplet spread on the target.

3. Were targets missed entirely, or coverage is consistent but sparse? The operator is likely not using enough water, and/or the spray quality is too fine. It has been demonstrated time and again that higher volumes improve coverage, but operators can try any of the options listed previously for partially-obstructed coverage. All the reasoning is the same.

Conclusion

Spraying fungicides effectively requires an attentive sprayer operator. Timing and product choice are very important, but when it is time to spray the sprayer operator should diagnose coverage with water-sensitive paper, and be willing to make changes to the sprayer set-up to reflect changing conditions. Thanks to the OBGA for sharing the survey data.

June 27, 2016

The Case for Low-Drift Sprays
This article was written by Tom Wolf for “PEI Potato News Magazine”, a publication of the Prince Edward Island Potato Board (http://peipotato.org/). It is reprinted with permission.
PEI Potato News Magazine
“Should I be using low-drift nozzles?” It seems like a simple question with an obvious answer. We all want to reduce spray drift, and this easy-to-use technology is the fastest way to get there.
And yet, the question is more complicated than it first appears. Yes, all applicators want to reduce drift, but many worry about the coarse sprays produced by low-drift nozzles. As a spray volume is divided into coarser (i.e. larger) droplets, there are fewer of them, and that can reduce coverage. It’s a legitimate concern.
Let’s start with our shared value first – the desire to reduce spray drift.
Given the economic, environmental and health impacts of spray drift, the importance is hard to over-state. That’s why spray drift management is a primary concern of our federal regulators whose job is to protect the public interest. It’s also a concern for the neighbours who have a right to keep unwanted products off their property, whether it’s residential or agricultural.
Conventional flat fan nozzles (XR8004) operating at 40 psi
Glyphosate drift with 20 km/h side wind, XR8004 40 psi
Low-drift nozzles (TD11004) operating at 60 psi
Glyphosate drift with 20 km/h side wind, TD11004 60 psi
For these reason, managing drift should be a foremost concern for applicators. The technology is vital to the crop production industry, and if we don’t take care of the issue, someone else will take care of it for us. That’s not the best path.
Much has been written about how to reduce drift. The key points are:
- choosing days with better weather,
- lowering booms and travel speeds,
- watching spray pressure,
- protecting the spray with shields,
- using coarser spray qualities on the whole.
Of these, the most economical and practical is using coarser sprays via low-drift nozzles. Engineered to emit fewer fine droplets, they are proven to reduce drift by anywhere from 50 to 95% compared to a standard flat fan of the same size. When it comes to reducing drift, they work.
When these tips first hit the mainstream as “pre-orifice” nozzles in the late 1980s, and later as “venturi” nozzles in the mid 1990s, we were impressed with their ability to reduce drift. And the obvious question was, what about product efficacy? Can fewer, larger droplets do the job? The answer, to our initial surprise, was yes.
In the late 1990s, the crop protection industry (including governments, universities, and the private sector), participated in studies throughout Europe, Australasia, and North America looking at low-drift spray performance. In Canada alone, we conducted over 100 studies and concluded that pesticide efficacy was not harmed when a properly adjusted low-drift nozzle was used. A surprising result showed that fungicides did not seem to need finer sprays, contrary to popular opinion, as long as water volumes were sufficient to provide adequate coverage.
As we did more and more studies, it became apparent which points were critical:
- When using venturi nozzles, spray pressure had to be increased from the industry standard of 40 psi to about 70 psi. This is because of a venturi nozzle’s two-stage design. The high pressure compensated for an internal pressure drop inside the nozzle. Sprays remained low-drift, but patterns and overall efficacy were better at this higher pressure.
Spray pattern of conventional spray (XR8002, 40 psi)
Spray pattern of low-drift spray (ULD12002, 60 psi)
Spray deposit of conventional spray (XR8002, 40 psi. ~10 gpa)
Spray deposit of low-drift spray (ULD12002, 60 psi, ~10 gpa)
- Spray pattern overlap needed to be greater with low-drift sprays – a full 100%. In other words, the edge of one nozzle’s spray pattern should reach the middle of the adjacent nozzles’ patterns. The pattern width at target height was now twice the nozzle spacing and this ensured good distribution of not only the spray volume, but droplet numbers, along the boom.
- We needed to pay attention to the target plant architecture and leaf surface properties. Plants such as grasses (with vertical surfaces and difficult-to-wet leaves) often had less spray retention with coarser sprays. Low-drift nozzles worked, but we couldn’t go as coarse in these cases. Careful selection of low-drift nozzles as well as more attention paid to operating pressure solved these issues.
- Our minimum water volumes had to increase slightly to compensate for the fewer drops produced by low-drift sprays. This was especially true for contact modes of action where too few droplets-per-area reduced performance. Using an Extremely Coarse spray at a very low water volume was asking for trouble.
Much of my efforts in recent years have been to advise applicators just how coarse they can safely go without harming product performance. This involves things we’ve touched on in this article, like water volumes, modes of action in the tank mix, target plant or canopy architecture, growing conditions, and the like. We’ve arrived at a few rules of thumb, like those above, but as always, it’s dangerous to oversimplify and there are always new situations to grapple with.
While we were learning how to tweak low drift nozzles to get them to perform, we also learned there were significant advantages to using coarser spray qualities.
1. Foremost, there was an immediate reduction in drift. One applicator told me years ago that switching to a low-drift spray removed a huge burden of worry from him, and that alone was worth it.
2. Low-drift sprays made it easier to spray on-time, even if weather conditions were marginal for conventional sprays. The result: the timely removal of weeds, or the correct staging of fungicides and insecticides. This has paid large dividends in terms of protected yield.
3. Coarser sprays can protect product performance from some adverse conditions, such as days with high evaporation rates. On such days, fine sprays evaporate to dryness so quickly that uptake can be limited. Larger drops stay liquid longer, with more uptake the result.
4. Directed sprays, be they banded sprays or twin fan nozzles for fungicides, make more sense from coarser nozzles. The reason is that these coarser sprays go where they’re pointed, whereas fine sprays lose their path in wind or through travel-induced deflection, very quickly.
5. We also learned about the air-entrainment that coarser sprays can produce. Large droplets dragged air with them, and smaller droplets could hitch a ride in their wake. This provided a form of air-assistance that reduced drift and carried small droplets into the canopy. Finer sprays had a harder time producing this type of drag, and sustaining it in the canopy.
When we analyzed the droplet size spectrum of coarse and fine sprays, we confirmed that the total number of droplets produced by any given volume of water had been reduced. Not a surprise. But two things struck us.
First, even though the average size of droplets in coarse sprays were very large, they still contained a population of small droplets. In fact, if you counted every single droplet in the spray, the vast majority were small and they were still taking care of coverage.
Second, the critical amount of coverage (measured as the percent of the surface area covered by spray deposits) that was necessary for a given product to work was lower than what we’d been aiming for. In other words, we didn’t need as much coverage as we thought we did, and any excess didn’t actually add to product performance in most cases.
We later analyzed the relationship between spray coverage and herbicide performance and found that the uniformity of the deposits was actually more important than the amount of coverage per se. So, if we focussed on proper overlap and spray pressure there was greater benefit than increased coverage alone. Deposit uniformity has become our research focus of late.
So, should you be using low-drift nozzles? By adopting the changes in pressure, overlap, and water volume outlined above, and paying more attention to the plant architecture and pesticide mode of action, we’ve been very successful in implementing low-drift sprays in all field crops. In my view, we can safely retire Fine sprays for all field crop pesticides. This means conventional flat fan nozzles, hollow cone nozzles, and the like. Get rid of them. All they do is add drift potential.
It’s safe to adopt low-drift sprays. Research and experience from the field prove that they work. Low-drift sprays should be viewed as an agronomic tool that improves application timing and accuracy. And with less drift, we show that agricultural practice can be both efficient and environmentally responsible. That’s going to be a very important story to tell, now and in the future.
May 20, 2016
Strategies to Spray the Top of a Perennial Canopy
Orchardists, nurserymen and hop growers share something in common – they want to get spray to the top of a tall plant canopy with as little waste as possible. The tops of trees, for example, are a primary site of infection as they filter spores from the air, so fungicide coverage is critical. Spraying the tops of high canopies (e.g. too high for over-the-row style sprayers) can be a difficult proposition.
Here are a few considerations:
- Wind moving through a planting, as a general rule, is twice as fast at the top of a canopy as it is at the ground. Wind carries spray off target.
- The further the distance a droplet travels, the smaller it gets as it evaporates and the less momentum it has. The likelihood of it hitting the target is greatly reduced.
- The top of a canopy typically has far less plant material than the rest of the canopy. Relatively speaking, there’s not much there to hit.
In order to overcome these challenges, the traditional axial orchard sprayer is nozzled with a larger proportion of spray distributed at the top of the boom. The idea is to increase the odds of some spray making it to the top of the canopy. Often, full-cone nozzles are used to accomplish this. Of course, if an estimated 10% of the spray actually impinges on the top of the canopy, the rest goes… well, somewhere else. This shotgun approach is hardly an efficient use of chemical.
Another strategy is to crank the PTO rpm’s up to 540, throw the fan in high gear and blow the spray as high as possible. The problem is, by increasing air speed and volume to carry spray to the top, the rest of the canopy (far closer to the sprayer) gets overblown and spray shoots right through. Some overspray might hit the next row, but most ends up on the alley floor. If you doubt it, consider how white your pant-legs get when you walk an orchard after spraying kaolin clay.
Others, mistakenly, might elect to raise the operating pressure to >150 psi in the hope that pressure will drive the droplets in a straight line at higher speeds. Most airblast sprayers using hollow cone patterns create very fine spray quality, even at 100 psi. Raising pressure means the droplets get even smaller, and tiny droplets have very little momentum. Increasing pressure just makes the problem worse.
Here’s what we propose.
Deflectors
If using an axial sprayer, employ air deflectors at the top of the air outlet to channel air (and spray) more effectively. The commercially-available deflectors are often just flat sheets, and air hits the surface and spills over all edges. Image pouring water onto a dinner plate – it just splashes over any which way. Better to replace those deflectors with a set that feature side-walls to channel the air. Anyone with access to a break and some sheet metal can make their own, but ensure they do not stick out beyond the wheel of the sprayer or they could snag plants and trellises. Always aim to overshoot the canopy top by a small factor to compensate for unexpected gusts of wind – better to overshoot a bit than to miss.
Commercial deflectors may or may not have channeling side walls. Inset: Homemade deflectors can do a great job.
The original Munckhof deflectors were reversed, and a larger set of extensions were fabricated and attached.
Towers
Better than deflectors, some sprayers move the air and nozzles closer to the target via ducted tower assemblies. They work very well, but they must be as tall as the target you intend to spray. Even then, an uneven alley can cause them to rock and you might still miss some upper targets. Operators using adjustable towers or ducts might angle them back to aim the air (and spray) on a slight upward angle rather than parallel to the ground, and that can compensate for a slight height difference, but it begins to defeat the purpose.
Nozzle body on upper tower deflectors. Still some air assist and a good idea, but use air induction nozzles.
Extra Nozzle Bodies
Some creative operators have attached additional nozzle bodies to the tower’s top deflector plate to aim it up in the top of the canopy. Still others have extended the wet boom itself higher than the tower. Unfortunately, although the nozzle is closer to the target (good) the benefit of air assist has been greatly reduced (bad). Air induction nozzles might help on boom extensions, per below.
Wet booms can be extended to reach high canopies, but may no longer benefit from air assist. Consider using air induction nozzles in these positions.
Air Induction Nozzles
Consider using air induction nozzles in the top two positions of each boom (totaling four per sprayer), with or without towers. There are three advantages:
1. Coarser droplets have more mass. They move in straight lines and are less likely to be deflected by wind before they reach the target.
2. Coarser droplets can be propelled by pressure, so unlike finer droplets they rely less on being carried by sprayer air.
3. Coarser droplets that miss the target do not continue upwards; they fall back out of the air into the orchard, reducing off-target drift potential.
No matter which strategy, or combination of strategies, you use to hit the top of the canopy, always confirm coverage using water-sensitive paper. Further, recognize that it’s very difficult to compete with high winds, so know when to wait it out.
Controlling your spray at the top of the canopy means better coverage and less waste. Plus, people won’t see this (wait until the ~50 second mark).
April 18, 2016
Exploding Sprayer Myths (ep.5): Early Morning Spraying
Should you really get up before the birds to spray? There are a lot of good reasons for early morning and night spraying, but if you’re in a strong inversion, you might be in a world of hurt. Here’s episode 5 in our series of short, educational and irreverent videos made with Real Agriculture.
February 8, 2016

When is Fungicide Coverage Critical? Always!

Introduction

A local strawberry producer was just beginning his harvest when the entire field was suddenly stricken with anthracnose. He would have done almost anything to save it, but he could only watch in frustration as the disease quickly devastated his crop. While he was telling me this story, he was wringing his hands; I’m sure he didn’t realize he was doing it. It had been more than a month since the crop was lost and he was obviously still very upset. Let’s put on our deerstalker hats and consider what might have caused the trouble.

Strawberry anthracnose. Photo by Pam Fisher, OMAFRA. — Strawberry anthracnose. Photo by Pam Fisher, former berry specialist with OMAFRA.

Most of the fungicides we apply in horticulture are protectants, not curatives. What that means is that the fungicide has to be in place before disease has a chance to take hold. Once it establishes a beachhead, you can typically only hold it at bay, not eradicate it. So, if you’re guilty of waiting too long between fungicide applications, the problems may have already begun. This is exacerbated when you don’t achieve the necessary spray coverage. Put the two together and mix in rainy and warm conditions and diseases like anthracnose can spread at alarming speed.

Method

I focus on the sprayer part of disease management, so I have to assume that inoculum is being controlled as much as possible (e.g. culling infected plants, drip irrigation, etc.). I asked the grower about his sprayer and his spraying schedule. He admitted to pushing the limits between fungicide applications, and being uncertain about the spray coverage he was achieving with his conventional flat fan nozzles.

In cases like this I try to find gentle ways of introducing the idea of using more water, increasing the frequency of applications, or buying new nozzles, because there is time and expense involved and many growers don’t want to hear that. However, when I started my soft sell routine, he looked me straight in the eye and said he’d lost tens of thousands of dollars in revenue so a few nozzles or a couple more applications were not a pressing concern. There’s a point in any endeavour when you’ve committed so much time and money that you’ll do pretty much anything to see it come to fruition (pun intended). He was willing to do whatever it took. This was my kind of guy.

So, in preparation for next year, we diagnosed spray coverage from five different sprayer set ups. Let me point out, as I always do, that spray coverage analysis does not necessarily extend to control. They correlate well, but if you aren’t using the right product or your timing is off, even the best coverage won’t help you. Caveats aside, here’s what we tested:

Setup1:

Broadcast application using a horizontal boom with TeeJet Twinjet 8006’s at 8.3 bar (120 psi) on 50 cm (20 in) centres. We calculated a nozzle rate of 3.9 L/min (1.03 gpm), so at 5.0 km/h (3.1 mph) that’s 923 L/ha (98.7 g/ac).

Setup 2:

Banded application on a horizontal boom equipped with a row kits suspending three TeeJet XR 8002’s at 8.3 bar (120 psi). We angled the two side nozzles so the fans were not perpendicular or parallel with ground. This kept more spray on the raised row and out of the alleys. The swath covered 50 cm (18 in) and we calculated a nozzle rate of 1.29 L/min (0.34 gpm), so at 5.0 km/h (3.1 mph) that’s 1,016 L/ha (108.6 g/ac).

Setup 3:

Banded application on a horizontal boom equipped with a row kits suspending three TeeJet XR 8002’s at 6.2 bar (90 psi). We angled the two side nozzles so the fans were not perpendicular or parallel with ground. This kept more spray on the raised row and out of the alleys. The swath covered 50 cm (18 in) and we calculated a nozzle rate of 1.14 L/min (0.3 gpm), so at 5.0 km/h (3.1 mph) that’s 896 L/ha (95.8 g/ac).

Setup 4:

Broadcast application using a horizontal boom with TeeJet Twinjet 8004’s at 6.2 bar (90 psi) on 38 cm (15 in) centres. We calculated a nozzle rate of 2.27 L/min (0.6 gpm) so at 5.0 km/h (3.1 mph) that’s 717 L/ha (76.5 g/ac).

Set up 5:

Broadcast application using a horizontal boom with TeeJet Twinjet 8006’s at 6.2 bar (90 psi) on 38 cm (15 in) centres. We calculated a nozzle rate of 3.4 L/min (0.9 gpm) so at 5.0 km/h (3.1 mph) that’s 1,076 L/ha (115 g/ac).

Protocol and Conditions

It was late September, so the weather was a cool 8 °C, humidity was low and winds averaged 5 to 15 km/h. We timed our passes to correspond with lighter wind wherever possible. Three sets of water-sensitive paper were placed in a single row, but only one pass was made per sprayer setup. One paper was placed at the top of the canopy which is always very easy to hit, so we oriented it sensitive-face-down. The second paper was placed midway down the canopy, oriented facing up. The final paper was also oriented facing up, but placed at the very bottom of the canopy, more or less on the ground. Collectively, we spanned the depth of the canopy.

Following each application, papers were collected for digital analysis using “DepositScan” which determines the percent of the paper covered with spray, and the droplet density. Both of these factors contribute to overall coverage. This wasn’t intended to be a rigorous experiment, so the means are presented here with standard error for the sake of comparison. There was no statistical analysis. In the case of papers located face-down, when only trace amounts of spray were discernible they were assigned a percent coverage of 1% and droplet density of 25 droplets/cm².

Results

A few observations before we get to the results. Research has demonstrated that row kits and higher volumes improve spray coverage, and that’s why we tried banding the applications using row kits in Setups 2 and 3. However, this grower didn’t use GPS to plant his rows, and while they weren’t too crooked, they made it challenging to apply in a band. Further, there is some concern that a banded application would miss any inoculum in the alleys. These are important points to factor in when considering methods to control disease.

The keen reader might notice we sprayed using pressures that exceed the manufacturer’s recommendations. In fact, none of these tips were rated over 60 psi and I used a formula to calculate their output at our high pressures. I have been heard to say (many times) never to exceed the manufacturer’s rates because it makes a mess out of the spray quality: droplets get much finer and pressure does not cause finer drops to penetrate a dense canopy. Better to switch to larger nozzles and stay within the pressures indicated on the manufacturer’s rate tables. I maintain that assertion. However, the grower was assured by fellow growers and custom applicators that this was the way to go and he wanted to try it. So, that’s where Setups 1, 4 and 5 came from.

Be aware that a small sprayer like the one in this study needs considerable pump capacity to support such high pressure and flow to the boom and maintain effective agitation. For more information on pumps, check out this article.

The following table expresses the coverage obtained by setup:

Set up	Paper Position	Mean % Coverage (±SE)	Mean Deposits/cm² (±SE)
Setup 1 – Broadcast XR 8006’s on 20” centres at 120 psi for 98.7 gpa	Top	1.0 ± 0	25.0 ± 0
	Middle	23.6 ± 4.5	253.5 ± 72.9
	Bottom	15.2 ± 2.1	423.2 ± 35.3
Setup 2 – Three banded XR 8002’s at 120 psi for 108.6 gpa	Top	2.1 ± 1.1	78.9 ± 53.9
	Middle	54.8 ± 12.1	275.2 ± 145.3
	Bottom	29.1 ± 2.7	544.5 ± 70.4
Setup 3 – Three banded XR 8002’s at 90 psi for 95.8 gpa	Top	7.4 ± 5.9	134.4 ± 52.2
	Middle	31.6 ± 15.9	203.6 ± 108.5
	Bottom	8.1 ± 3.9	224.4 ± 102.3
Setup 4 – Broadcast Twinjet 8004’s on 15” centres at 90 psi for 76.5 gpa	Top	1.0 ± 0	25.0 ± 0
	Middle	33.3 ± 5.0	240.7 ± 70.9
	Bottom	12.9 ± 6.0	263.9 ± 95.2
Setup 5 – Broadcast Twinjet 8006’s on 15” centres at 90 psi for 115 gpa	Top	2.3 ± 1.3	105.6 ± 80.6
	Middle	48.9 ± 5.5	194.3 ± 25.6
	Bottom	19.5 ± 10.3	246.8 ± 40.4

The results may be easier to compare and contrast in the following graph.

Strawberry coverage results for all five setups.

Observations

According to the results, Setup 2 appeared to provide the best overall coverage. This is no surprise given that it was the second highest volume and employed a row kit. This corresponds with findings that have been published elsewhere. However, the excessively high pressure did create a lot of drift and the row kit didn’t always line up with the planted row. Further still, there’s the potential for missing anything that might harbour inoculum in the alleys, like runners. This setup wasn’t appropriate for this particular situation.

The second-best overall coverage was obtained from Setup 5. This represented the highest volume, and a preferably lower pressure on twinjets, which may have allowed the spray to penetrate the canopy from multiple angles. This broadcast application is more reliable for hitting meandering rows and covers the alleys as well. So, the grower plans to employ this setup for the 2016 season, spraying at shorter intervals and confirming his coverage with water-sensitive paper. Let’s hope it works out.

End-of-Season Update

The grower that volunteered his time to this study has reported that his strawberries at the end of the 2016 season were absolutely beautiful. Granted, it is always difficult to draw a direct correlation between sprayer calibration and control. For example, 2016 was a very dry growing season and disease pressure was lower than in 2015. Nevertheless, spray coverage plays an important role in crop protection and our work to improve sprayer performance definitely played it’s part. His success is great news!

November 2, 2015