Extensive spray equipment experiments last summer in Ohio showed that three different tools consistently performed better than others at delivering liquid sprays to the lower leaf canopy, where Asian soybean rust begins. The top three tools included an air-assisted sprayer, a premixed air and liquid sprayer and a conventional spray boom with a mechanical “canopy opener.” (For more information about results from Part I of the study, see “Rust Stop Ahead,” December 2005, page 6.)

Although not complete yet, the initial data from a second part of the study now confirms many of the findings from the first part, says Erdal Ozkan, Ohio State University (OSU) Extension agricultural engineer.

“The air-assist sprayer and the canopy opener are still the two best tools,” he says. “The twin-flow spray pattern continued to do poorly, and the trends established in the first study generally remained consistent in the second study. The only thing that reversed significantly was that one sprayer was either second- or third-best during our initial measurements using artificial targets, but now it's on the bottom of the results.”

Knowing what spray application equipment is most effective at reaching the lower plant canopy is important, given the nature of soybean rust.

“The disease starts from the lower part of the canopy and progresses toward the top,” explains Ozkan. “By the time rust would arrive, say in Ohio, soybean plants may be in advanced growth stages, sometimes 3-4 ft. tall with good canopy coverage. Penetrating the lower part of the canopy with applications is very challenging.”

Although a wide variety of fungicides are available to control soybean rust, he adds, “fungicide labels currently fail to clearly specify the type of application equipment or setup that would provide the best coverage.”

Even though more time consuming and expensive to conduct than Part I, Part II of the study helped to verify the accuracy of using artificial spray targets to record and measure spray deposits. For the first part of the study, researchers sprayed water and dye onto soybean fields where artificial targets (sheet metal plates and water sensitive paper) could record how much product and what type of spray coverage reached the lower leaf canopy. In contrast, Part II of the study relied on actual plant leaves to measure spray deposits from Headline fungicide.

“Water sensitive cards and plates don't always behave the same as plant leaves,” says Ozkan, who collaborated with researchers Heping Zhu and Richard Derksen from the USDA Agricultural Research Service in Wooster, and Anne Dorrance and Dennis Mills from OSU's Plant Pathology Department to conduct the experiments. “Artificial targets are more rigid,” says Ozkan, “and they don't flutter in the wind as much, for example, as real leaves.”

Researchers analyzed leaf samples from 20 individual plants in each of four replicated plots during the second part of the study for each treatment. Technicians ground the leaves and used a liquid chromatography/mass spectrometer analyzer to detect and quantify the active ingredient in Headline, pyraclostrobin.

“Fungicide residue analysis from leaf samples taken from the lower level of the canopy confirms that mechanically opening the canopy and using air-assisted delivery produced higher spray deposits in the lower canopy area than nozzles used on a conventional, broadcast sprayer,” says Ozkan. “Additional chemical analysis will be conducted on leaves taken from the middle canopy area and stem samples taken from the lower and middle canopy areas. We hope to finish with analyses of these samples by the end of May.”

Ozkan says he doesn't expect these additional analyses to differ greatly from those already conducted on the lower leaves, but they might help to further explain what happened to cause the change in performance of a premixed air and liquid sprayer.

“Actually, everything that we mentioned in our conclusions of the first study are true in the second study, except that something went wrong with one sprayer,” says Ozkan. “We want to see the results for the middle canopy to decide for sure what it was.”

Funding for the research was provided by the Ohio Soybean Council through the soybean checkoff, Ohio State University Extension, OARDC, USDA-ARS, BASF Corporation, The Corn And Soybean Digest, Gregson Technologies, Jacto, Inc., Spraying Systems Co. and Unverferth Equipment Co.

Spray Results Summary

Results from a recent USDA Agricultural Research Service/Ohio State University study conducted in Wooster, OH, show which equipment and spray application methods would likely work best to control Asian soybean rust. The following are some highlights from the study:

  • When using conventional sprayers — nozzles that provide medium spray quality rather than fine or coarse — tend to provide a better penetration of droplets inside the plant canopy and better leaf coverage.

  • Spray hitting the target from two different angles may produce better coverage if the canopy is not dense. But in dense canopy conditions, flat-fan nozzles with a single spray pattern producing medium quality spray tend to provide a better penetration of droplets inside the canopy.

  • Air-assisted sprayers did a better job with penetration of droplets and spray coverage than a conventional sprayer.

  • Using a mechanical canopy opener, conventional sprayers may provide coverage and penetration nearly as good as those from air-assisted sprayers. However, the canopy opener will not be effective in reducing spray drift, which is achieved using air-assisted sprayers.

  • At 15 gal./acre and 7 mph, and under dense canopy conditions, flat-fan nozzles provided better coverage and penetration into the canopy than hollow-cone nozzles.

The spray application recommendations are not limited to soybean rust, however. Ozkan says they are also applicable for managing soybean aphid or other soybean diseases, such as white mold, that exhibit similar growth characteristics to soybean rust. To find a more detailed report on this study, visit the following OSU Web site: http://fabe.osu.edu/FACULTY/Report_Rust_study_2005.pdf.