The difference between the correct droplet size for spraying a herbicide and spraying a fungicide is the width of a human hair. That miniscule margin of error can mean the difference between effective application and calling your crop insurance agent.

In a typical herbicide application, producers are looking for a droplet size of around 300-500 microns — approximately the same width as three to five of the hairs on your head. For a soybean rust fungicide application, experts want those droplets at around 200-300 microns.

“Droplet size is going to be the critical factor in spraying for soybean rust,” says Scott Bretthauer, application technology specialist with the University of Illinois. “Farmers need to select a nozzle type, orifice size and operating pressure that will give them the desired droplet size.”

  1. Nozzle type: There are several different shapes and sizes that have an effect on droplet size. If choosing between a flat-fan or hollow-cone nozzle, experts say flat-fans have better drift reduction at the pressures needed to apply fungicides for soybean rust.

    “The problem with a hollow-cone nozzle is it produces a lot of small droplets; less than 150 microns,” says Bretthauer. “If droplets are too small, they're drift-prone.”

    When choosing between a twin and a conventional nozzle, Bretthauer recommends twin-spray nozzles. “A nozzle where one flat-fan is angled forward and the other backward — instead of one nozzle shooting straight down — will get greater canopy penetration,” he says.

  2. Orifice size: The second choice you'll face is orifice size and type. The size needed can be determined by looking at a manufacturer's catalog and choosing the right output for your spraying speed and pressure, says Robert Wolf, an application technology specialist for Kansas State University.

    When applying a rust fungicide, you'll need nozzles that will produce fine- to medium-sized droplets based on the American Society of Agricultural Engineers scale (see chart below). For example, to meet this specification Wolf says there may be as many as 20 different nozzle types, orifice sizes and pressure combinations to achieve a “typical” fungicide application — 20 gal./acre at 10 mph. Wolf is currently working on canopy penetration trials to sort out which of the 20 combinations may provide better coverage.

    You'll also need to choose whether or not to use a drift-reduction nozzle.

    “For applications other than fungicides for soybean rust, there's a lot to gain by switching to a pre-orifice low-drift nozzle,” says Erdal Ozkan, an Ohio State University aricultural engineer.

    However, with rust fungicide applications we can't make this assumption. For rust fungicide applications droplets must be small enough to find their way to the lower parts of the canopy, says Ozkan. Unfortunately, low-drift nozzles were not designed to produce small- to medium-size droplets. But, he explains, if the sprayer can handle higher pressures, eventually at a certain pressure level some of the low-drift nozzles may actually provide spray quality that may be categorized as fine to medium.

    “The key point when using a low-drift nozzle for rust fungicide application is that first you need to check the charts in the nozzle catalog to find out at what pressure that particular low-drift nozzle can provide fine- to medium-spray quality,” he says. “Then you have to make sure all the components of your sprayer will function well at that pressure.”

    Just because nozzles can handle the pressure, doesn't mean your spray pump can. The increased pressure could cause your pump to fail. Some pumps, especially centrifugal pumps, aren't equipped to run at high pressures and can actually decrease flow rate when running at low rpms, Ozkan says.

  3. Operating pressure: If nozzle choice is the key to droplet size, then operating pressure is the lock.

    “The appropriate pressure will deliver the right droplet size at the application rate you're trying to achieve,” says Mark Hanna, agricultural engineer at Iowa State University (ISU).

    Herbicides are generally applied at 5-10 gal./acre. However, manufacturers recommend applying most soybean rust fungicides much higher — at 15-20 gal./acre. Your choice of nozzle affects pressure. “You'll generally get smaller droplets at greater pressures,” Hanna says.

  4. Drift: With the talk about smaller droplets and higher pressure, farmers might shrink droplets too far and cause spray drift.

    There is a point where droplets are too small, says Wolf.

    “I liken it to the difference between shooting a cannonball and shooting a cannon full of feathers,” he says. “Most people understand that the feathers are too light to go anywhere, they'll just get carried off by the wind. If your drops are too small they won't penetrate the canopy, they'll just float off landing in the field on top of the canopy without penetration.”

    Drift accross property lines should be avoided whether it affects somebody's plants or not, says ISU's Hanna. “Just because plants weren't killed doesn't mean drift didn't occur. You could still get fined.”

    The good news is that rust fungicides shouldn't damage most crops, Hanna says. But, he warns, be careful if you're next to an orchard or a vineyard. There are indications that rust fungicides can cause damage to grapes and apples.

    From a cost standpoint, growers should be concerned about how drift can affect their bottom line, says Ozkan. “If 20% of the material drifts away, there's 20% less material to fight soybean rust,” he says.

  5. Canopy penetration: Effectively controlling soybean rust is all about canopy penetration, according to Illinois' Bretthauer. After droplet size, sprayer boom height, nozzle overlap and sprayer type are the other important keys to getting good canopy penetration, he says.

    Boom height is determined by the target height. On herbicide applications, the target height is fairly easy to determine — it's the height of the weeds, says Bretthauer. However, with soybean rust, the equation gets a little trickier. You're not only trying to cover the whole canopy horizontally, but since soybean rust spreads from the bottom of the plant, you've got to cover the plants vertically as well.

    “If you set the boom too low, you won't get enough overlap from your nozzles and you'll get streaking in the top of the canopy,” Bretthauer explains.

    To increase canopy penetration, he recommends increasing the angle at which spray hits the canopy. If you're using 80 fan angles, he suggests spraying with 110 tips instead. “The wider angle you can get, the better. And the wider the droplet trajectory, the lower you can set the boom so there's less chance for droplets to drift,” he adds.

    While any type of ground sprayer will work, research shows that air-assist sprayers have performed the best in canopy penetration studies. Visit ohioline.osu.edu/aex-fact/pdf/0526.pdf for results.

    Spray banding in 30-in. rows is also a possibility, says ISU's Hanna. But if you don't already own the equipment, it's also unlikely that you'll be willing to buy just for rust, he says.

    Another possibility is aerial application. “There's a lot of misunderstanding about aerial application,” says KSU's Wolf. “Ground spraying or aerial application should work equally well for rust, provided they're both set up correctly.”

    Wolf says aerial applicators may have some advantages over ground spraying — they won't have to worry about tracking rust spores from field to field, they can cover more acres in a timely fashion and they wouldn't be hindered by wet or muddy field conditions.

  6. Tankmixing: With the possibility of spraying herbicides, insecticides and fungicides, growers are looking for ways to eliminate trips across their field. You can tankmix fungicides with insecticides (provided the label allows it), but not with herbicides. It has little to do with chemistry compatibility, but everything to do with application compatibility, says Wolf.

    “You can't tankmix herbicides because of the droplet size differences. If you're going to put glyphosate on and you match the fungicide droplet size, then you're making the glyphosate driftable. That's a problem,” says Wolf. “But if you put the fungicide on at the proper glyphosate droplet size, then you might not get the coverage you need for soybean rust. Either way, it's an expensive adventure. You need to apply them separately.”

    Insecticides and fungicides will work together because they have similar droplet size needs, says Wolf. He is planning some field trials to verify tankmix possibilities.

  7. Calibration: Overall, to spray smart you need to be prepared, says Wolf. To do that, you need to calibrate your sprayer.

“Sit down at your kitchen table with a nozzle catalog and a calculator. Plug in all your application scenarios — herbicide, insecticide, fungicide — different gallons per acre, travel speeds and nozzle spacings,” says Wolf. “Do the math (gal./acre × mph × nozzle spacing ÷ 5,940).

Find the right orifice size in the catalog and then the only decision you have to make is the type of nozzle to use.” An additional step for fungicide applications is to study the spray quality charts to make sure the selection meets the 200-300 micron size required.

From your sprayer, collect the nozzle output in a container. The output should equal the gal./min. you've already calculated, he says.

If you're using the 1/128 of an acre calibration method (see “7 Easy Steps To Calibrate Your Sprayer,” page 8, April 2005) you can also precalibrate your sprayer. Test nozzles to see if they produce the proper flow rate at the pressure that will give you fine- to medium-spray quality, says Ozkan.

He encourages farmers to set up a marker the same distance in the field (1/128 of an acre) as they marked in their earlier calibration. Then make sure to cover the distance within the same time frame to get the same gallons per acre application rate.

Spray Quality Categories
ASAE Standard S-572
Category Color Microns
Very Fine (VF) Red < 182
Fine (F) Orange 183-280
Medium (M) Yellow 281-429
Coarse (C) Blue 430-531
Very Coarse (VC) Green 532-655
Extremely Coarse (XC) White >656