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Fungicide programs can hammer Asian Soybean Rust

CLEVELAND, MISS. - Delta farmers filled the conference room at the Delta Ag Expo, seeking answers on how to avoid or stop a threat that looms over their soybean crop.

But Monte Miles, plant pathologist with the USDA’s Agricultural Research Service at the University of Illinois and a leading expert on Asian Soybean Rust, told the audience that fool-proof antidotes do not exist.

“If you have good conditions to grow soybeans then you have the perfect conditions to have Asian Soybean Rust,” Miles told attendees at the long-running farm show in Cleveland, Miss., on Jan. 18.

ASR is an airborne spore disease that leapfrogged north of the equator from Columbia to various pockets of the U.S. during four days of intense hurricane weather in September. The disease, already commonplace in Brazil, Japan, Australia, India, Nigeria and other countries, has been blamed for widespread soybean crop destruction.


U.S. plant pathologists thus far have been unable to locate any one gene that can fully resist the disease but are hoping to incorporate global ARS resistance models made by peers – particularly in Brazil, which identified ASR on 95 percent of its acreage in 2004.

Miles conveyed hope, saying the disease – if found active in a particular field – can be managed effectively with minimum yield loss on the condition that farmers diligently adhere to one of two resistant methodologies: preventative and curative programs comprised of fungicides.

But, he emphasized that results from either program are predicated on aggressive ASR scouting, preparedness and timely intervention.

“If you’re scouting from your pickup you’re never going to see this disease. You must be out in the field,” he said, noting that the fast spreading disease is evidenced by small chloridic dots that eventually can burn up the entire leaf.

Quarantine exemption

Miles said while fertilizers, tillage practices, higher/lower plant populations and wide row spacing have not prompted resistance, beyond seasonal freezing and drought, only fungicides have been proven to impede ASR.

Presently, there are four U.S. registered fungicides available for ASR application: two cholorthalonil products (Bravo and Echo) and two strobilurin products (Quadris and Headline). Additional fungicide compositions, Miles hinted, could soon be available through the states’ emergency Section 18 Quarantine Exemption Request. Those include:

  • Popinconazole
  • Tebuconazole
  • Myclobutanil
  • Propiconazole
  • Trifolxystrobin
  • Pyracolstrobin
  • Pyraclostrobin plus boscalid
Two may do

Because every farmer’s needs are unique, combined with the fact that much more domestic information needs to understood on ASR, the USDA does not offer specific application recommendations. Miles did say that farmers using fungicide programs should plan on using at least two different product applications, based on studies done in Brazil.

“Plan on two applications. If you don’t need it, don’t use it. But if you need it you better have it in place,” he said.

“Label recommendations right now (suggest) 14 to 20 days between applications. And fungicide needs to penetrate the canopy. You can have the best product but it must penetrate. The first product you choose will drive what your second product will be. Don’t use the same one twice (consecutively).”

Alan Blaine, Extension soybean specialist at Mississippi State University, said the next two years would be the roughest for soybean farmers until scientists can determine the most effective benchmarks for ASR resistance. He said an effort is under way to formulate a best-advice reference for farmers – one he hopes will be published in the next few months.

Miles said researches have identified about 800 variety trials out of a 20,000 pool that they believe may still hold promise for, one day, producing a resistant variety.

“This is a real tough disease to scout. It will be a real challenge to stay ahead of it.”

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