Top Crop Manager

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Maximizing fungicide use

Managing fungicide application with modern approaches.


June 30, 2020
By Presented by Tom Wolf, Agrimetrix Research and Training, at the Top Crop Manager Plant Health Summit, February 25-26, 2020, Saskatoon.

Topics

First of all, herbicide spraying is easy. There are a large number of modes of action available. Weeds are targeted in wide open spaces with no canopy covering the weed. On the other hand, spraying fungicides into denser canopies is a challenge. Solving this challenge is quite elusive. There are no silver bullets.

We have done a couple dozen studies in a lab environment with actual crop canopies, and monitored where the spray goes in the canopy – the top, middle or bottom. By the time the spray reaches mid-canopy, about 40 per cent of the spray is reaching into the canopy. At the bottom of the canopy, just a fifth of the spray is left. With fungicides, that’s often where the target is. When we look at the variability of the deposit in the canopy, the bottom of the canopy has the highest variability. So, these two challenges, diminishing the dose and increasing its variability as the spray moves into the canopy can really work against us in the fungicide world.

Maximizing fungicide efficacy depends on what principles you are trying to achieve.

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It is important to know your target. Fungicides may require whole plant coverage, but in some cases there is a very specific target. Fusarium head blight is one of those, and it’s at the top of the canopy. In many of our pulse diseases, the target is further down.

Knowing the mode of action is also important. Typically, in the fungicide world, most of the popular modes of action are “locally systemic,” which means they move in the xylem. This means they don’t move towards the growing point. Therefore, the spray must target the part of the plant that requires protection.

A principle that I want to start with is counterintuitive, because it is commonly thought that a finer spray works better with a fungicide because they provide better coverage. But I would like everyone to start with a coarse spray. There’s no harm in using a finer spray, but the reason I’d really like to debunk that whole myth is that there is value in using a coarser spray, and that value has to do with the timing of the operation. Coarse sprays provide a larger window of application, and that is important because spray timing is, in fact, the most critical component of disease control.

After selecting a nozzle that delivers a coarse spray, spray coverage can be fine-tuned with water volume, spray pressure and nozzle angle. This allows modification of the droplet numbers to suit specific situations.

A challenge with disease control is that fungicide labels have generic, feel-good statements that say things like, “Ensure adequate coverage,” or, “adequate and uniform coverage.” There’s only one case in the world of fungicides that I have found that is incredibly specific. It’s the Proline label for Fusarium head blight, where the label makes reference to the forward and backward orientation of nozzles to increase deposition on the wheat head.

What does ensuring good coverage mean? To me, it means measuring it. I like to use water sensitive papers as indicators. They’re easily available locally. Water sensitive paper can help measure droplets per unit area. Percent area covered can be calculated. These measurements infer what is really important. They imply the dose per target or coverage. They can also teach us something about uniformity. Are we getting the same result over and over again in different parts of the canopy or different parts of the field? Uniformity is a very, very important part of efficacy.

There are a couple of tools that can help with measuring spray coverage. In Australia, the GRDC funded the development of an app called SnapCard. With this app, you take a picture of water sensitive paper, and it analyzes an area and provides percent coverage. It quantifies it, and it’s repeatable.

A second tool is from Brazil. DropScope is a scanner that hooks up to your computer or mobile device with a USB port or Bluetooth connection. It reads a one-inch by three-inch water sensitive paper and gives you five or more quantifiable parameters such as droplets per square centimetre, average droplet size, and per cent coverage.

Measuring spray quality
Spray quality is categorized as “fine,” “medium,” “coarse,” “very coarse,” “extremely coarse,” and “ultra-coarse.” For herbicides, spray quality has migrated from fine and medium sprays in the 1980s to extremely coarse and even ultra-coarse now in the era of dicamba herbicide protection against spray drift. Fungicide application has stayed more or less static with medium spray quality.

Fine spray quality may have a large proportion of the droplets as fine droplets – less than 150 micrometres. These fine droplets provide coverage, but they also drift and evaporate. That limits your window of opportunity in using these droplets effectively.

Extremely coarse sprays, on the other hand, have exactly the opposite composition of droplet sizes. They have a large proportion of extremely coarse droplets – greater than 500 micrometres in size. They’re large, but are they providing coverage? They might even be bouncing off the target.

The middle ground has been coarse sprays that provide a reasonable amount of the dosage – 20 per cent in this example – in fine droplets that provide coverage and a relatively small amount of extremely coarse droplets that might not do anything. One of the big moves over the years has been the migration from a medium spray, which drifts too much, to a coarse spray, which is good for the industry in terms of stewardship and also agronomically.

Sclerotinia control
I conducted research on sclerotinia control in canola with Randy Kutcher when he was with AAFC at Melfort. For sclerotinia control, the primary target for fungicide application is the petals and buds in the top of the canopy. But there is a secondary target that may be important as well, and that is the leaf axil further down in the canopy where the saprophytes decompose the flower petal.

A fine/medium spray from a conventional flat fan nozzle and a coarse/very coarse spray from a TurboDrop air induction tip were compared at 40 and 80 psi, along with a very fine spray produced by a hollow cone nozzle. The research had five site years in the Melfort area in the early 2000s, and found that the two fungicides that were applied significantly reduced sclerotinia stem rot incidence compared to the untreated control. But there was no statistical difference in control between the different spray qualities in the field trials.

In the lab, dye was sprayed onto a canopy of canola. The plants were partitioned into top, medium, and bottom parts and then further separated into petals, buds, leaves, and stems. The parts were washed in solvent, and the dye analyzed to determine the spray deposition on the various parts.

Looking at the total amount of spray retained by each plant, the proportion of spray dye retained by the important targets, petals and buds, did not differ with application methods. This is good news because it means that coarse sprays can be used for sclerotinia control. And there is value in that because you can spray with less drift, under a wider range of conditions, with a better chance of being environmentally friendly.

The lab study looked at the effective boom height comparing 20- and 30-inch heights. Spray deposition did not vary between the heights in the upper, mid and lower parts of the plant.

Three spray pressures of 20, 40 and 60 psi were also compared for spray deposition. Spray pressure is interesting because, traditionally, people believe they can force the spray into the canopy by applying a very high pressure. The fact is that small drops slow down to their terminal velocity within a few inches of leaving the nozzle. In this research, again, there was no difference between the three pressures in deposition in the upper, mid and lower parts of the plant. It was quite clear that increasing spray pressure did not result in greater deposition lower in the canopy.

Based on the research, the ideal application for sclerotinia stem rot control isn’t that different from what’s currently being done. Applying at the correct crop stage has always been important. Single or twin nozzles both work, but there’s really no need to go to twin nozzles in this situation. Coarse to very coarse sprays are going to be adequate. There’s no harm done going to a medium spray, but there’s probably nothing to be gained. A slower travel speed of 10 to 12 miles per hour (mph) is recommended because it is conducive to more uniform application. The boom can be run at medium-to-low height of 24 to 30 inches. I believe any fungicide application should use at least 10 gallons per acre (GPA) – 10 to 15 GPA is the benchmark. High water volumes are a function of crop density. A canola crop growing five to six feet tall could have the flowering region extend well into that canopy, so higher water volumes are important for better coverage.

Fusarium head blight control
With Fusarium head blight the primary target is the head, while the secondary target is the flag and penultimate leaf. Research in the lab looked at spray coverage of the head. However, one important factor missing in the lab is wind, so this research shows the best-case scenario.

A single nozzle moving at slow speed of eight kilometres per hour deposited about two-thirds of its spray on the forward side of the vertical target. Almost one-third of the spray was deposited on the backward side of the head. This was likely from wrapping around to the other side by the smallest droplets.

With a double nozzle at eight km/h, spray is directed forward and backward. The same spray volume is still applied. The double nozzle added deposition to the rearward-facing side of the target with most of that from the backward-facing nozzle. That’s very important because fungicides don’t translocate within the wheat head. Both sides of the targeted head have to be hit with the fungicide if you’re going to be successful.

Research also found that greater angled spray produced better results. Air induction double tips set at 30 degrees from vertical with a coarse spray traveling at 15 km/h produced variable results with 79 per cent of the coverage on the front of the target. When the double nozzles were set at 60 degrees from vertical, the wider angles added spray to both the front and the rear side compared to the narrower spray.

Coarser sprays deposited more on the forward side of the heads than a medium spray. Coarse droplets retain their trajectory, and with the nozzles pointed forward and backwards, the droplets fly in that direction. Finer sprays lose that trajectory.

Probably the most important recommendation is to maintain a low boom height. We compared three boom heights of 30 cm (12 inches), 45 cm (18 inches) and 75 cm (30 inches) with fine, medium and coarse sprays with double nozzles. Moving from low to medium to high boom height resulted in the overall deposit being cut in half. That’s strictly a function of the droplets still moving forward and backward at the low boom height, but they are no longer doing that with the higher boom heights. There was still a benefit of a coarser spray at all three heights.

Travel speed also had an effect on spray deposition on the wheat head target. Fusarium spray application is the only situation where a faster travel speed consistently provides a better deposit. Moving from five mph to 10 mph increased deposition at the low, medium and high boom heights. Droplets being forced into a forward motion improve the deposition on the vertical target.

Overall, the ideal application for Fusarium head blight control is to apply fungicides with a low boom set at 20 to 24 inches above the canopy, twin nozzles with coarse to very coarse spray quality, travel speed of 10 to 12 mph, and 10 to 15 GPA water volume.

Pulse crop application
In research with Sabine Banniza at the University of Saskatchewan College of Agriculture and funded by Saskatchewan Pulse Growers, fungicide application was assessed in pulse crops. A conventional flat fan nozzle with medium spray quality, an air-induced nozzle with coarse spray, and a twin nozzle that produced a fine spray quality were compared. Water volumes of 10, 20, and 30 GPA were compared with a conventional 8001 nozzle at varying travel speeds. Partitioning of the spray in the different parts of the plant was assessed.

Surprisingly, the nozzle type didn’t have much impact on spray deposit in chickpeas. Fine, medium and coarse droplets all had similar deposition on the lower, middle and upper parts of the plants.

The impact of water volume on spray deposition varied by canopy type. On kabuli-type chickpeas with a more open unifoliate canopy, there was no real advantage of applying more water to get through the canopy. But with desi chickpea with a denser, more closed canopy, there was a strong effect with more droplets deposited in the lower canopy with increasing water volume.

When you can’t see the target that you’re trying to hit with a spray, that’s when increased water volume starts to pay dividends. When lentils were all the rage about three or four years ago, many people sprayed their lentils three times, and the third application, when that canopy was closed, was indeed at 30 GPA.

I did some research with an industrial partner and PAMI, and looked at the distribution of the spray in field pea. We looked at spray deposition in the top, middle and bottom of the peas, and at five, 10, and 15 GPA using water sensitive paper. Coverage in the top of the canopy was fairly good at five GPA, but better at higher water volumes. In the mid canopy, droplet density was starting to go down. But at the bottom of the canopy, the droplets are getting very sparse with a low water volume but are possibly still acceptable with the highest water volume.

When you look at deposit density, according to Syngenta’s instructions on water sensitive paper, 80 drops per square centimetre should be the minimum target when using a fungicide. Five GPA didn’t meet that objective; partially met it with 10 gallons, the 15 GPA met that target more completely on field pea.

Improving spray efficiencies
Spray timing is absolutely crucial in helping fungicides work well. With some diseases, the spray window is tight – one or two days – depending on the weather. There is an opportunity cost for not spraying on time. If you have a quarter section and you lose $20 per acre because of that three or four-day delay resulting in higher disease, that hour that it would’ve taken to spray that field is worth $20 x 160. That’s $3,200. That’s why a lot of the work that we’re doing on Sprayers 101 with Jason Deveau is looking at how to get more hours of spray time.

The answer is to improve the efficiency of the operations when you’re not spraying. That means to fill faster, clean faster, and adjust the spraying operation so that you spend more time spraying. An analysis of eight John Deere R4045 sprayers found that when the sprayer engine was running, 45 per cent of the time the equipment was spraying, 33 per cent it was idling (cleaning and filling), and 22 per cent was spent in transport.

The single biggest efficiency improvement that we’ve seen in spraying in the last 10 years is the three-inch pump. Spray time is incredibly valuable and improving efficiencies can pay off.

Charts courtesy of Wolf & Banniza, 2004.

Aerial application
In the aerial application world water volume is a constraint that has to be dealt with. Smaller droplets must be used, or you have to be satisfied with lower droplet density. But low water volume and finer spray risks less coverage and more drift.

With aerial application, droplet density is simply lower because of fine droplets. In fact, it’s difficult to make large droplets from an aircraft because of shear atomization. When too large of a droplet is emitted from an aircraft, there’s a secondary atomization to the air shear, and the big drops in air resistance just break apart, explode, and make a lot of small drops. A lot of counterintuitive things happen in the spray world.

Rotary atomizers are commonly used with aerial applicators. They have adjustable vanes that adjust their rotational speed, and that adjusts their droplet size. They form a deposit that is actually not bad.

We have to be mindful of the variability that’s inherent in aerial spray. There are regions under all aircraft where the deposit is different from adjacent regions. It is an ongoing challenge.

Saskatchewan Pulse Growers funded a study at Saskatoon that I again collaborated on with Sabine Banniza. It compared ground and aerial applications applied within one-half hour of each other targeting aschochyta in chickpea. We did two Headline and two Lance applications.

Sabine did the disease ratings and the disease became progressively worse over the growing season. Fungicide application significantly reduced the disease severity. Ground and aerial application were statistically similar except at the last rating on August 15, but in all likelihood yield would have been determined by then.

Weigh wagon results showed the value of using fungicides to control aschochyta. Yields with fungicide application were almost triple the untreated check. Aerial was slightly less yielding but not statistically significantly. I think that’s very important, and that you can have confidence in aerial application for disease control.

More rotary-winged aircraft are being used in Saskatchewan. At normal flying speeds they are more productive, as they turn faster than a fixed wing aircraft. Even though they don’t have a large tank or a wide boom or a very fast travel speed, they can be more productive. If rotary-winged aircraft fly at slower speeds, the rotary wash can push the spray further into the canopy.

One of the application methods not used on the prairies is air assist. Jason Deveau studies air blast spraying in trees. Air is used to transport small droplets into the canopy. That technology isn’t available for field crops, but perhaps a future aerial application method can deliver that for us.