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Scouting is key: Fungicides can slow Asian rust

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

But Monte Miles, plant pathologist with 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.

Asian soybean rust is an airborne spore disease that leapfrogged north of the equator from Columbia to various pockets of the United States 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 Asian soybean rust resistance models made by peers — particularly in Brazil, which identified Asian soybean rust 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 methodologies: preventive and curative programs comprised of fungicides.

But, he emphasized, results from either program are predicated on aggressive Asian soybean rust 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 Asian soybean rust.

Presently, there are four U.S. registered fungicides available for Asian soybean rust 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

“Maximum economic return”

Miles said because Asian soybean rust travels randomly — yet is capable of moving as far as 500 miles at any one instance — a farmer's decision to select a preventive course of action or to take a wait-and-see attitude depends on other factors, especially the level of risk-taking. While investment costs may prove wasteful if Asian soybean rust doesn't strike, making no preventive applications can cost dearly if the rust is detected and curatives are applied too late.

Resistant fungicide products, applicators and adjutants cost, on average, between $10 and $24 per acre. He said it's up to farmers to evaluate cost benefits and weigh the choices. “If you have the disease in your field, you are limited to products. Your scouting will tell you which products you need to use,” he said.

“If you wait too late, it can be managed, the season is not wasted; then it can be controlled, but it might not be worth the trouble or the money.

“Whatever practice gives you maximum economic return on your farm, that is where you start.”

He said Asian soybean rust infection cycles happen rapidly, normally maturing between five to seven days from initial infection to the point when each lesion can start producing a new set of spores.

“‘What do I use?’ is a question I often get. But I can't answer that because I don't know enough about your farming practices,” Miles said. “What is your risk level? A protectant program may be a high cost but is a low risk.

“With a curative program, are you going to wait until the rust is in the county south of you or in the field across the road?”

Miles advocates that a farmer begin by becoming familiar with the state emergency exemption list, as well as each product's application rate and frequency. Test trials on the products reveal disparate results, he said, but while all show suppression, each one does so uniquely.

Test results, Miles added, reveal that the most effective resistance programs are applied in high volume, in the form of fine droplet sizes and via flat fan nozzles. The fungicides, he said, can be tank-mixed with some insecticides, though it's advised they not be applied with herbicides.

Two may do

Because each farmer's needs are unique and because more domestic information about Asian soybean rust is needed, USDA does not offer specific application recommendations. Miles said farmers using fungicide programs should plan to use at least two different products, 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 should have it in place,” he said.

“Label recommendations right now (suggest) 14 to 20 days between applications. The fungicide needs to penetrate the canopy. 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 Asian soybean rust 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 researchers have identified about 800 variety trials out of a 20,000 pool that they believe may hold promise for, one day, producing a resistant variety.

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

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