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Corn+Soybean Digest

What's in a fungicide's name? Soybean rust control




Farmers might need a pronunciation key to sound out the words. In the months to come the fungicide ingredient names are likely to roll off their tongues much easier, however, as producers prepare to plant the first soybean crop in the continental United States threatened by soybean rust.

"Fungicides will be a new experience for most soybean farmers in Indiana and throughout the Corn Belt," says Greg Shaner, a Purdue University Extension plant pathologist. "Most growers have never used a foliar fungicide, so there will be questions about what products are available, how to use them and when to use them for effective disease control."

Fungicide is the only effective treatment for controlling soybean rust, a fungal disease that can dramatically reduce a soybean crop's yields. The fungus, Phakopsora pachyrhizi, is spread from field to field by airborne spores. One month ago the USDA announced that rust had been found in a Louisiana soybean field. Since then, the USDA has confirmed the presence of rust in eight other states as far north as Missouri and Tennessee.

Experts believe Hurricane Ivan carried rust spores from South America to the mainland United States in September.

Shaner is among a team of plant pathologists from soybean-producing states working with the U.S. Environmental Protection Agency to secure emergency use status for fungicide ingredients not previously approved for use on soybeans. The "Section 18" requests have added four fungicide chemicals to the five that were already registered for soybean use through the EPA.

"Among the fungicides that are either currently approved or that we anticipate being approved, we have four different chemical groups," Shaner says. "Those represent four different modes of action.

"One reason for seeking registration for more fungicide materials was so that farmers would be able to use different modes of action. The reason for doing that is because if everybody was using the same kind of fungicide there's a good chance that the rust fungus would adapt to it and become resistant and, therefore, the fungicides wouldn't be effective."

Fungicides come in two basic types: those that form a protective barrier on the outside of the soybean plant and those that work from the inside out.

"Among the fungicides that will be used on soybean rust there's one group -- the chlorothalonil products -- which are strictly protectant. They just form a coating on the surface of the plant and are not absorbed by the plant," Shaner says.

"The other fungicides that either are labeled for use or are in the process of review, are what we call systemic. The plant absorbs them, but they don't move throughout the entire plant. They move toward the tips of leaves and they may move from leaves that are sprayed into new growth but they don't move down in the plant. So they're not truly systemic in the sense of when people take an antibiotic and it moves through the bloodstream throughout the body."

Fungicide ingredients approved or granted emergency use status for soybean rust in Indiana, and brand-name products that contain them, include:

  • Chlorothalonil (protectant) -- Bravo, Echo 720
  • Azoxystrobin (systemic) -- Quadris
  • Propiconazole (systemic) -- Tilt, PropiMax, Bumper
  • Tebuconazole (systemic) -- Folicur
  • Myclobutanil (systemic) -- Laredo
  • Pyraclostrobin (systemic) -- Headline
  • Tetraconazole (systemic) -- Domark
  • Propiconazole plus Trifloxystrobin (systemic) -- Stratego
  • Pyraclostrobin plus Boscalid (systemic) -- Pristine

"Systemic fungicides are a little better in the sense that if you don't have uniform coverage on the soybean plant when you make the initial application, the internal redistribution that you'll get will make up for some of those deficiencies," Shaner said. "On the other hand, with protectant fungicides if you only manage to cover 30 percent of the leaf area, then the other 70 percent is unprotected."

Not only will farmers be learning new chemical names and modes of action, but also new application skills. Applying fungicide is not the same as using herbicides and insecticides, Shaner says.

"Two areas will be different" than using other chemical inputs, Shaner says. "One is that for many herbicides, complete coverage of the weed isn't that critical. If you get a critical dose of herbicide on the weed the herbicide is absorbed and then it kills the plant. With fungicides, good coverage of the soybean leaves that are present at the time of application is very important. So there will be some issues about the droplet size and the ability of the spray to penetrate down into the canopy that will be a bit different from what farmers have been accustomed to in applying herbicides.

"The other critical issue in using fungicides against soybean rust is to get an application on when the disease first appears. If a farmer waits until there's enough rust that it really begins to look serious, it's too late."

Fungicide treatments, including product and application, are expected to cost farmers between $14/acre and $35/acre.

For more information about soybean rust, visit the Purdue Plant and Pest Diagnostic Laboratory soybean rust page at or the Purdue Agricultural Communication soybean rust page at

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