The mechanics of spraying fungicides for Asian soybean rust aren't open for debate, said Dennis Gardisser. That may come as a surprise to those who have heard conflicting advice at winter meetings. At one meeting, ground rigs are cited as the preferred means of spraying. At another, attendees are told aerial application is the only way to go.
“In the end, it makes no difference to me what type of delivery vehicle is used,” said Gardisser, Arkansas Extension agricultural engineering expert. “As long as the fungicide is sprayed uniformly and properly, it doesn't matter. The thing with rust is it begins in the bottom portions of the plant that are moist and shaded for the longest time.
“To control Asian soybean rust, we need to get as much (fungicide) material as possible down into the canopy. To achieve that, many will go with high volume ground applications with droplet sizes that are too large. If that happens, the material will end up on the ground and be of no value in controlling this disease.”
In treating Asian soybean rust in South America, the best success has come with a droplet diameter of about 280 microns. “There, they don't care if the product is put out by air or ground. The platform doesn't matter — helicopter, fixed wing, ground rig or a floating balloon.”
What does matter is the product must be very uniform in its distribution.
“We have many photographs of what happens when an area is treated lightly or missed. Rust will take over there. Timing and product selection are also key.”
A typical volume for South American aerial application is 20 liters per hectare (2 gallons per acre), said Gardisser. “Their agriculture is a little different than ours — it's huge in terms of scale and field size. What we recommend for aerial applications in the United States is 3 gallons to 7 gallons per acre.
“The scale slides according to how dense the canopy is. For example, 7 gallons would be used on a very tall crop that is completely closed in. Three gallons, on the other hand, would be for fields being treated earlier without as much canopy.
“The droplet we're recommending is 285 to 300 microns. For ground applications, we're recommending 10 gallons per acre for the sake of logistics.”
While South Americans use ground rigs, Gardisser said they avoid them if possible. One reason: certain areas of a field can be prone to develop rust.
“If you drive a ground rig through it, you can carry spores down the row. Or, commercial applicators can carry spores from farm to farm. Also, ground applicators are more difficult to get through a field when it's wet — and that's when we really need to treat. I've seen several photos of ground applicators sunk into deep mud.
“In Brazil, they're even treating with straight product at 2 quarts per acre. We won't recommend that. Of course, labels here won't allow it, either.”
Spraying fungicides is nothing new for many farmers in the United States.
“In the South, we've been putting fungicides on soybeans for years. We're also familiar with spraying for late blight in potatoes. Potatoes are similar to soybeans in canopy structure to a degree. When spraying potatoes, we've found a 300-micron droplet at 5 to 7 gallons per acre works very well.”
If spraying for Asian rust more than once, Gardisser suggests varying the treatment direction. That will provide a different penetration angle into the canopy. Second, use a nozzle — “twin flat fans” — with more than one penetration direction
“Spraying Systems has an excellent nozzle to use. That company has done a lot of solid research on this. We're directing Extension agents to their Web site (www.teejet.com) because probably 90 percent of the nozzles used to treat rust with ground applications will be manufactured by Spraying Systems.”
An analogy for getting good spray coverage is, “Every year my wife wants me to spray-paint a Christmas wreath. If I just go up and down with the spray can, it won't get completely covered. I have to angle the can in different directions. We're trying to do a similar thing when spraying fungicides for Asian soybean rust.”
Syngenta — “which is doing a lot of research in Brazil” — is recommending 5 gallons by air and 10 gallons by ground. “That's plenty, I think. If we try to put out 20 gallons, I think we'll make too big a droplet.”
Gardisser said speakers at several meetings have offered advice “I really disagree with. Some have said their plan is to increase pressure and blow spray straight into the canopy. That simply won't work.”
To show why, take a spray bottle with an adjustable nozzle and close it down tight. Now, pump on the trigger a few times very fast. The highest pressure you can generate produces a fog.
“Those droplets won't go anywhere. But if you open the nozzle up to produce a 285-micron droplet, you can soak someone across the room. We must target the proper droplet size.”
Gardisser said he hears several fallacies regularly. “One is ‘We must go with ground rigs because airplanes won't work.’ That isn't true at all.
“I also keep hearing, ‘You absolutely must have 20 gallons per acre and extremely high pressures.’ That isn't true either. Droplet size and uniformity are much more critical.”
An idea touted at several aerial applicator meetings also won't work, he warned. “Some call for flying really low in an effort to get the fungicide to mix into the canopy better. That doesn't happen, though. When you fly too low, it actually compresses a layer of air under the plane. That causes a distortion in the spraying pattern resulting in less uniformity.
“That compressed air must go somewhere, so it ends up rolling off the end of the wings. By flying so low you're throwing product up in the air that will be lost to evaporation, drift, whatever. There's an optimum height to spray at and that doesn't change whether you are putting out herbicides, fungicides or insecticides.”