You'd do well to have a plan in place in advance for how best to control Asian soybean rust, even if it's unlikely we'll see the disease in most fields this season.

“There's only about a six-day window of opportunity for stopping soybean rust when it first occurs,” warns Erdal Ozkan, The Ohio State University (OSU) Extension engineer. “Farmers need to have an action plan in place at the start of the season.”

Soybean rust symptoms typically appear on the lowest part of the plant and then progress toward the top.

Ozkan says, “Detecting the disease early and using the most effective control mechanism are keys to controlling this disease.”

Aside from early detection, another vital ingredient in the fight against soybean rust is applying a uniform fungicide treatment to the plant.

“How uniformly the fungicide is deposited on the spray target is as important as the amount deposited,” he says. “Applying too little fungicide results in poor control and reduced yields, while depositing too much wastes money and increases the risk of polluting the environment.”

To increase fungicide coverage, Ozkan advises two basic principles:

  1. Reduce spray droplet size, and/or

  2. Increase carrier (water and fungicide) volume.

“Extremely large droplets don't provide good coverage and may result in waste of chemical,” he explains. On the other hand, extremely small droplets can be drift-prone, he says, and should be avoided.

“For soybean rust control, we have to do everything possible to utilize the fine- to medium-sized droplets, approximately 200-300 microns in size,” says Ozkan. “Drift-prone droplets (those smaller than 100 microns) do not have any momentum pushing them into the canopy, and they don't last long after they are released from the nozzle.”

Low-drift, flat fan nozzles may still work to control drift while applying fungicides, he points out. However, they will need to operate at slightly higher pressures (60-70 psi), which would increase the number of medium-sized droplets but still avoid generating extremely fine droplets.

Proper nozzle selection helps to produce both the correct flow rate and droplet size, says Scott Bretthauer, University of Illinois Extension application technology specialist. He advises farmers to comb through nozzle manufacturer catalogs to find the appropriate nozzle for the job and the equipment available.

“An application rate of 15 gal./acre or higher is recommended for ground applications,” he says. “However, older sprayers may not be designed to meet flow rates and might have to go at a slower speed to achieve the correct rate and droplet spectrum.”

Still another way to potentially increase uniform fungicide coverage to the plant would be to tankmix a deposition aid, suggests Bob Wolf, Kansas State University Extension application technology specialist.

“A deposition aid will generally improve the amount of product deposited into the canopy,” he explains. “It's also likely that this could prove to be an affordable option for many growers when trying to control soybean rust.”

HIGH-TECH EQUIPMENT is the final option for boosting uniform fungicide coverage to the plant, but this equipment generally requires a significant added expense that may not be suitable for every farmer.

For example, research at OSU shows that compared to a conventional flat fan nozzle, a low-drift flat fan nozzle and a disc-core hollow cone nozzle, “an air-assisted sprayer is the best equipment option we have, if our goal is to achieve maximum coverage from a fungicide,” says Ozkan. “Unfortunately, a commercial-scale sprayer with air assistance may add from $10,000 to $15,000 to the price tag of the equipment.”

Still, for a commercial applicator or a large farmer, the one-time cost for the air assistance technology could outweigh the potential income lost to soybean rust in just one growing season, he says.

Ozkan, Wolf and Bretthauer also point towards new, pulse-width modulation technology that can influence application volumes (gallons per acre) through an electronically controlled solenoid valve. Unlike conventional sprayer systems, which typically regulate spray volume through pressure, speed or by changing to a different orifice size, pulse-width modulation can regulate spray volume through the valve.

Wolf says there are scenarios when it's vitally important to maintain flow rate while increasing droplet size to protect sensitive areas from spray drift and still provide uniform spray coverage to the intended target. He cites fields bordering subdivisions, waterways or high-value crops as examples, especially when those sensitive areas are downwind from the application.

On the other hand, pulse-width modulation technology is expensive, Bretthauer says adding, “The cost is about $140 per foot of boom.”

For most farmers who already own their own spray booms for applying herbicides, and who want to use the same equipment for applying fungicides, “the best bang for the buck is proper nozzle selection,” he adds. “The key (to proper fungicide application) is to create the droplet size that gives a good balance of coverage, penetration and deposition.”

Adding Fungicide Plus Herbicide May Double Trouble

Spraying requirements for fungicides and herbicides are typically quite different, says Bob Wolf, Kansas State University Extension application technology specialist. As a result, he says he is discouraging farmers and custom applicators who may want to combine weed and disease control into one pass.

For example, he says that some farmers have recently asked him if adding a fungicide to a tankmix with a postemergent herbicide, such as glyphosate, would work to control both weeds and Asian soybean rust. The goal, of course, is to save time and money by avoiding two separate trips through the field.

Although the intention may be good, the result probably won't be, warns Wolf.

“It's just not likely to be viable,” he says. “Roundup in particular works effectively with a larger droplet size, which also limits drift potential. So if farmers want good herbicide control, they would probably have to use droplets that are too big for effective fungicide application.”

Most systemic herbicides like Roundup can control weeds with good coverage on the upper leaf canopy, he explains. However, to adequately control Asian soybean rust, farmers would need a fungicide application that penetrates low into the leaf canopy, where the disease originates.

To obtain that low-canopy coverage, a spray applicator would need to select higher application volumes and a droplet size much lower than typically recommended for most postemergence herbicide applications, says Wolf. He reminds applicators that small droplet sizes are more prone to drift than large droplet sizes. Thus, he cautions against Roundup applications at very high pressure, which creates very fine droplets.

Mark Hanna, Iowa State University Extension engineer, agrees that a farmer's spray strategies need to be different when targeting weeds compared to diseases.

“Nozzle tips are particularly important this year with the increased interest in fungicide treatments for soybean rust,” he says. “Flat-fan tips or twin orifice nozzles are typically best for obtaining the smaller droplet size and the good coverage needed to apply fungicides effectively, but that's not what you want when you apply glyphosate, particularly where there are drift issues.”

More On Rust

Rust Tracks, which provides up-to-date information about Asian soybean rust, is sponsored by Syngenta Crop Protection. For more information about soybean rust, visit Syngenta's Web site, www.soybeanrust.com.