With new cases of Asian soybean rust being reported on an almost daily basis in the lower Southeast, it's important that growers know how to make the most effective treatment when and if a fungicide application becomes necessary.
Coverage of the soybean plant at bloom stage is critical for the prevention of Asian soybean rust, and research conducted in South America and in the United States can help growers set up their sprayers for optimum effectiveness when treating for the disease, says Paul Sumner, University of Georgia Extension engineer.
Soybean producers in Georgia's Coastal Plain were advised to begin spraying for Asian soybean rust on July 19, after samples taken from a sentinel plot at the University of Georgia Coastal Plain Experiment Station in Tifton were confirmed to be infected with the disease.
Extension specialists say they are unsure how the disease will progress, however, frequent thunderstorms in the area coupled with the reproductive growth stages of the crop necessitate protection with fungicides. Soybean rust also has been confirmed in a sentinel plot in southeast Mississippi, which is not far from confirmed rust cases in Baldwin County, Ala., where rust was found on both commercial and sentinel plots.
Rust is first established on the lower plant surfaces, says Sumner, so good fungicide coverage is needed on the lower leaves and stems. “The fungicide must get on the lower plant to protect it. The fungicides that are available for treatment of Asian soybean rust need surface coverage to be effective, and this is achieved best by smaller droplets,” he says.
The two types of chemistries currently available for rust are contact and locally systemic, he adds. “The contact fungicides remain on the surface and will protect only the area that is covered. The locally systemic material is absorbed into the plant tissue.
However, this is not the same type of systemic movement normally seen in systemic-based herbicides. The fungicide will not move throughout the plant, but only within a small absorption zone, such as a single leaf,” says Sumner.
For maximum plant surface coverage, Sumner recommends that growers use a nozzle that develops medium to fine droplets (225 to 325 microns). Some fine droplets are acceptable, he says, but they drift easily and may blow over a dense canopy, while coarse sprays drift less, but make fewer droplets available for coverage.
“The extended range and low drift fan nozzles can achieve good coverage when operated at pressures above 50 psi,” notes Sumner. “At this pressure, spray is forced down into the canopy. Care should be taken not to operate your sprayer above 75 psi with these nozzles, as a large amount of fine droplets will be generated,” he says.
Sumner recommends that growers consult nozzle manufacturer catalogues for droplet classification at the various pressure settings and nozzle sizes. “Total spray volume applied should be 15 to 20 gallons per acre for ground application. This will give sufficient droplets to cover the target area. Ground speed of five to seven miles per hour is desirable,” he says.
The boom height, he says, should provide 50-percent overlap. Keep it as low as possible to give you better penetration, coverage, and to lessen drift potential, he says. If available, air-assisted sprayers, such as those used on vegetables, will do an excellent job of coverage using the same application rates, he adds.
Aerial applicators also should use a nozzle that will produce a medium droplet size (285 to 235 microns), says Sumner, with a spray volume of five to seven gallons per acre.
Two excellent publications on sprayer setup have been developed specifically for Asian soybean rust, he says. They include “Spraying Recommendations for Soybean Rust,” Ohio State University Extension, http://www.tifton.uga.edu/spray/ohio0526.pdf, and “Aerial Application: Tips for Rust Control,” University of Arkansas, http://www.tifton.uga.edu/spray/Rust%20Arkansas.pdf.
Soybean producers who suspect that a rust infection is nearby, or already has occurred, should include a triazole or triazole-strobilurin fungicide in their program, say University of Georgia Extension specialists. They advise growers to continue to follow the progress or lack of progress of the disease before making a second fungicide application. Should a second application of fungicide be necessary, it could be applied as early as two to three weeks after the first application.
Rust symptoms, they say, will appear first on the lower leaves of the canopy. The first symptoms of rust are very small brown to brick-red spots in a non-descript yellow spot on the upper leaf surface. Eventually, slightly raised pustules form in the spots, primarily on the lower leaf surface. As pustules become numerous, leaves turn yellow and drop prematurely.
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