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College of Food, Agricultural, and Environmental Sciences


Brazil Experiences with Rust Offer Insight for U.S. Growers

March 28, 2005

Editor's note: This is part of a periodic series on information regarding soybean rust. The goal is to provide media with the latest updates on the disease and Ohio State's role in research and education. These updates are expected to continue throughout 2005.

WOOSTER, Ohio — First-hand observations of soybean rust in Brazilian soybean fields are telling Ohio State University plant pathologists one thing: that with diligent scouting, timing of fungicide applications and a blessing from Mother Nature, the disease is easily manageable.

Anne Dorrance, an Ohio State research plant pathologist with the Ohio Agricultural Research and Development Center, was part of a nationwide envoy of soybean pathologists and agronomists who recently traveled to Brazil to observe the impact of soybean rust on the country's crop. The purpose of the trip was to get a better handle on what U.S. soybean growers might expect from the disease as it is anticipated to spread northward.

"The recurring theme throughout the trip was that the fungicides work. We saw humungous fields in Brazil and the only place where we could find active rust lesions were in the check plots and in the skips of the sprayers," said Dorrance. "U.S. producers will actually have more resources available than the Brazilians to assist in tracking and monitoring soybean rust, and to assist in the best timing of fungicide applications."

Soybean rust, an aggressive disease that attacks the crop from the bottom-up, was identified in eight states late last year. Dorrance said that the lessons learned in Brazil will help U.S. growers become more knowledgeable regarding the behavior of the disease, preventive maintenance and effective control in the event soybean rust were to continue spreading northward into such states as Kentucky, Ohio and Indiana.

Some take-home tips that were revealed during the trip to Brazil include:

• Timing of fungicide applications is key to limiting losses. "We are hoping U.S. growers can get away with as few as one application because it's going to be the most affordable," said Dorrance. "That being said, the tighter the application window to the appearance of the disease, the best and more effective use growers will get out of that fungicide." Each of the three types of fungicides that are available (strobilurin, triazole and chloronitrile) has a certain shelf life. The longer the window between application and the disease, the less effective the fungicide is.

• Accuracy of fungicide applications is critical to limiting disease development. "Soybean rust attacks from the bottom-up. It's something we have yet to get a clear sense as to why. It could be because of a physiological aspect of the plant, or it could be because the environment is favorable for disease development," said Dorrance. "If you've ever walked through a soybean field in the morning, you come out with your pants soaking wet. The dew lasts longer, humidity lasts longer, it's a nice and cool. It's a very favorable environment for the fungi and it's where we need to be looking and where we need to be hitting the applications. And I'm not talking about good coverage or excellent coverage, but outstanding coverage."

• The development and spread of soybean rust is highly weather dependent. "One location we visited in Brazil hadn't had rain for about two weeks. The plants were doing just fine and under the dry weather conditions the disease had stopped," said Dorrance.

• From flowering to full seed will be the critical time to protect plants. "We were finding the disease in Brazil on plants that were in mid-flowering to setting pods. Yet, we really had to dig to find the lesions," said Dorrance. "This is a situation that we may see in Ohio."

• Rust can be a challenge to diagnose. "Rust lesions look similar to brown spot and downy mildew. Only when the lesions began sporulating were we able to diagnose it as soybean rust. When rust begins producing spores there is nothing else out there like it," said Dorrance.

Luckily, Ohio growers may not have to concerns themselves with correctly diagnosing the disease. Thirty sentinel plots (sponsored by check-off support through the United Soybean Board North Central Soybean Research Program and the Ohio Soybean Council, and coordinated by Ohio State and the Ohio Department of Agriculture) have been set up throughout Ohio to help diagnose soybean rust. In the sentinel plots, nearly every leaf will be monitored throughout the season either on a weekly or bi-weekly basis. The first diagnosis of soybean rust in Ohio would most likely come from these sentinel plots.

Candace Pollock
Anne Dorrance