Winning the War Against Soybean Rust Requires a Strategy

January 19, 2005

COLUMBUS, Ohio — The best weapon growers have to fighting soybean rust is fungicides, but an effective job of controlling the disease is questionable without an application strategy.

Erdal Ozkan, an Ohio State University Extension agricultural engineer, said that how uniformly a fungicide is applied to the plant is just as important as applying the right amount —both of which can be achieved by using the right equipment at the right time and as accurately as possible.

"Spraying the right amount of fungicide is not enough to achieve effective control. The single most important factor affecting the outcome of the fight against this disease is to get a thorough coverage of the plant with the fungicide. Research has shown that there is a very strong correlation between coverage and efficacy," said Ozkan. "We have the technology to achieve this, but it may come with a higher equipment cost. Our farmers have been somewhat conservative when it comes to adopting new spray technology, but soybean rust may change this habit because the alternative to not changing equipment may be the complete loss of the crop."

Asian soybean rust (Phakopsora pachyrhizi) was discovered in the United States in late 2004 in Louisiana soybean fields. Since then, the disease — which can spread quickly and devastate soybean fields if not effectively controlled — has been confirmed in Alabama, Georgia, Florida, Mississippi, Arkansas, Tennessee, South Carolina and Missouri. Speculation remains as to whether the disease will make an appearance in Ohio, but growers are being encouraged to be prepared nonetheless.

"There are no soybean varieties currently available that have high levels of resistance to soybean rust. This leaves us with only one alternative option: do as good a job as we can with spraying fungicides that are registered to control this disease," said Ozkan.

The researcher, a professor with the Department of Food, Agricultural, and Biological Engineering, has offered some tips to help achieve the best coverage when spraying for soybean rust.

• Calibrate sprayers frequently to make sure the right amount of chemical recommended on the label is applied. "Too little fungicide results in poor control and reduced yields, while too much wastes dollars and increases the risk of polluting the environment," said Ozkan.

• Keep spray volume above 15 gpa (gallons per acre) for best results.

• Choose the appropriate size and type of nozzles and operate them at a pressure that will allow them to produce small to medium-size droplets (200-300 microns).

• Choose "low-drift" nozzles, which allow a grower to increase the pressure without increasing the number of small, drift-prone droplets (those at 100 microns or smaller). Operate these nozzles at slightly higher pressures (60-70 psi) than usual. Higher spray pressures usually help the droplets penetrate the canopy better.

• Use directed spraying, if applicable, to improve coverage.

• Use twin nozzle/pattern technology. Research has shown that two nozzles, one angled forward and one angled backward — known as a flat-fan pattern — perform better than single nozzles spraying in one direction. Nozzles producing a cone pattern are not recommended for soybean rust.

• If economically feasible, use air-assisted spraying. Research has shown that air-assisted spraying, which uses air to help droplets (100 microns or smaller) reach the underside of the plant canopy, consistently provides the best coverage and droplet penetration, even when beans are at or near their full-growth stage.

"Soybean rust first shows its symptoms usually in the lower parts of the plant and works itself up towards the top of the plant. Detecting the disease early and using the most effective control mechanism are keys to controlling this disease. Complete coverage of the disease could be even more challenging if the symptoms of the disease are found at later stages of plant growth when the plant is close to having the full canopy," said Ozkan. "Air-assisted spraying may help in facing those challenges."

A commercial-scale sprayer with the air assistance may add from $10,000 to $15,0000 to the price tag of the equipment, but Ozkan reminds growers that cost may well outweigh the income lost due to soybean rust in one growing season.

Candace Pollock
Erdal Ozkan