The ability to successfully apply fungicides will be critical in dealing with an outbreak of soybean rust during the coming growing season. Although further research is needed to determine the best application procedures, results from work done on other crops and from practical experience in South America can provide some useful guidelines, according to Scott Bretthauer, application technology specialist with University of Illinois Extension.

"The most important factor to control when spraying for soybean rust will be spray droplet size," Bretthauer said. "Small spray droplets provide better coverage and tend to deposit well on the target, but if droplets are too small they will be unable to penetrate and deposit in the canopy, or they may drift off target."

He notes that large droplets will not deposit as well because they have a tendency to bounce or run off the plant, and there will be fewer droplets which reduces the coverage.

"The key is to create the droplet size that gives a good balance of coverage, penetration, and deposition," Bretthauer said. "A nozzle produces a range of droplet sizes known as the droplet size spectrum. A medium droplet spectrum is recommended for making applications to control soybean rust."

Bretthauer points out that the droplet spectrum can be controlled by selecting and correctly using the proper nozzle. The droplet spectrum created by nozzle is a function of the nozzle design, the size of the opening, and the operating pressure.

"In general, nozzles with large openings produce larger spray droplets, while nozzles with smaller openings produce smaller spray droplets," he said. "As pressure is increased, smaller droplets are created. Using low pressures reduces the amount of small spray droplets."

Bretthauer suggests using a nozzle manufacturer's catalog to select a nozzle size and operating pressure that will create the desired droplet spectrum of medium as well as deliver the required nozzle flow rate in gallons per minute.

"An application rate of 15 gallons per acre or higher is recommended for ground applications, but don't fail to create the right droplet spectrum when applying at higher rates," Bretthauer said. "If necessary, use a slower sprayer speed to achieve the correct rate and droplet spectrum."

Another important consideration is the style of nozzle tips used to spray for control of soybean rust.

"Although hollow cone nozzles can be used to make fungicide applications, they create a great deal of very small droplets that will not penetrate dense plant canopies," Bretthauer said. "Extended-range flat-fan nozzles create smaller droplets in the higher end of their pressure operating range, but applicators should be sure not to exceed the upper pressure limit."

While air-induction nozzles are often considered only as drift control nozzles, there are several designs that produce fine and medium droplet spectrums. Research in peanut canopies has shown that air-induction nozzles can provide good canopy penetration.

"Air-induction nozzles are designed to work at higher pressure than other flat-fan nozzle designs, and should be operated according to the manufacturers recommendations," Bretthauer said. "The research in peanuts also showed twin spray nozzles work well for achieving good canopy penetration and target coverage."

Twin spray nozzles produce two flat-fan patterns, one angled forward and the other angled backward. There are two designs available: single tips with two openings and modified caps that hold two individual nozzle tips.

"With these caps, the applicator can chose the type of nozzle to use, such as an air-induction tip," Bretthauer said. "Sprayers with air-assisted booms have been shown to provide excellent canopy penetration and spray deposition. It is important to match the air flow rate to the canopy as not to increase the risk of drift."

He notes that aerial applications have given excellent control of soybean rust in South America. By changing deflector angle or orientation of an aerial nozzle, the droplet size spectrum can be altered.

Bretthauer points to the Aerial Spray Nozzle Models developed by the USDA Agricultural Research Service's Aerial Application Technology Research Team at College Station, Texas as a valuable tool that aerial applicators can
use in setting up their aircraft.

"By entering the nozzle type, orifice size, nozzle or deflector angle, pressure, and air speed, the model calculates the droplet spectrum and other valuable information," he said. "This allows an aerial applicator to set up the aircraft to create the droplet spectrum required for a job. Good control of droplet size is one of the reasons agricultural aircraft can successfully make soybean rust applications at five gallons per acre."

He adds that agricultural aircraft also have the advantage of speed and the ability to spray when field conditions are too wet for a ground sprayer. While keeping the boom as low as possible works well for ground applicators, Bretthauer notes that flying too low can actually increase drift and reduce deposition for aerial applicators.

"An ideal height for aircraft to fly is often between 10 and 14 feet above the canopy," he said. "Any higher and the droplets are excessively exposed to wind. At lower heights, droplets can become trapped and carried off in
air turbulence caused by the aircraft flying so close to the crop canopy. Whether using aerial or ground applications, it is important to follow the proper guidelines as closely as possible."