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Corn+Soybean Digest

Be Ready For Rust | Soybean Rust Could Affect Soybean Acres if Conditions are Right in 2010

Although it remained pretty much south last year, soybean rust (SBR) had its way with hundreds of thousands of soybean acres, providing a glimpse of how it can blow into the heart of the Corn Belt if the perfect storm emerges in 2010.

In 2009, 16 states and over 400 counties had SBR on either soybeans or kudzu. “I don’t think SBR should affect northern growers’ management decisions now, but I do think they should pay attention to rust activity in the South during the season,” says Daren Mueller, Iowa State University plant pathologist.

“(In 2009) we had about 1 million soybean acres planted late in June (due to wet weather),” adds Scott Monfort, University of Arkansas plant pathologist. “Of those, 600,000 had to be sprayed with a triazole fungicide for SBR.”

Monfort’s lab in Lonoke, AR, tested over 1,000 samples of kudzu and soybeans. Each sample contained 100 suspect bean leaves. They literally examined every leaf under a microscope; a lot of SBR was found.

“But we’re starting from scratch this year because there won’t be a rust problem unless it blows in,” says Monfort. “From what we saw last year, there’s always a potential. We had one-third of the crop planted June 1 and on.”

SBR is triggered by tropical winds and moisture blowing up from Mexico, Texas and the Deep South. Tropical storms can expedite SBR movement. Symptoms may include tiny brown or brick-red dots on the upper leaf surface and pustules or bumps on the lower leaf surface.

Monfort says his group needed to look at so many samples because “these dots are very hard to see in the field when the disease is at low levels. And this is when you want to first find SBR. Once it is established in a field for a long time, the symptoms can be seen in the field.”

SBR can reduce yield, especially if it hits early in the season, says Mueller. If SBR is detected prior to the R5 growth stage, fungicide treatment is recommended.

Fungicides used to control SBR are classified as triazole or strobilurin. The triazoles include Tilt, PropiMax, Alto, Caramba, Laredo, Bumper, Folicur, Orius, Uppercut and Domark. Strobilurin fungicides will usually stop both spore germination and host penetration if applied as a preventative treatment. Strobis include Headline or Quadris. There are also a couple of fungicides that are premixes of triazoles and strobilurins.

Should you spray without SBR infestations? Probably not, unless a field is likely to face other disease problems early. Mueller’s recommendation is usually to wait and not spray unnecessarily, but other factors can play into a grower’s situation. Cost of a triazole is $15-30/acre, he adds.

“If you purchase fungicides early in the year you may receive cost discounts for both the product and the application, which will help your chances in getting an economic response,” he says. “However, we continue to see the most consistent economic responses when foliar diseases are present and they can’t be determined until the season starts. This puts growers in a tough situation to decide if or when they should purchase fungicides.”

Monfort says SBR can impact yield up to the R6 stage, but that once beans reach the R5.5 stage, they will likely out-run damage from SBR if disease pressure is light.

“About 50-60% of our beans get sprayed with fungicide every year,” says Monfort. “Most is for other diseases. In all likelihood, if you’re spraying for frogeye and rhizoctonia aerial blight between R3 and R5, you would usually receive some protection against SBR depending on the application timing and the arrival of SBR.”

With the end of federal grants supporting the sentinel plot system, funding will drop 60-80% in 2010 compared to its peak, says Don Hershman, Extension plant pathologist at the University of Kentucky, who coordinated the first four years of the sentinel plot effort.

Sentinel plot monitoring in the South will be limited to April 1 through Oct. 15, says the North Central Soybean Research Program (NCSRP). In the North, “we only need to know where soybean rust is from about April 1,” Hershman says. “If models indicate there could be a problem, there will be ad-hoc monitoring.”

For updates, see USDA’s SBR Web site.

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