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Corn+Soybean Digest

Rust Relief

With Asian soybean rust a greater threat every year since it blew into the U.S., growers are looking for ways to counter potential rust rampages. Mississippi farmer Tim Clements believes earlier maturing varieties are among the answers.

And if fungicide treatments are needed, studies by the Southern Soybean Research Program (SSRP) indicate that if rust is present in the R1 and R3 stage, a strobilurin fungicide should be sufficient if the risk is moderate. If the risk of rust is high, then a triazole fungicide must be used, or a combination of triazole and strobilurin.

Purdue University Plant Pathologist Greg Shaner says spray recommendations for when and what to spray are similar in the Midwest and southern production areas, even though spraying may be needed sooner down South.

“In the absence of any direct experience in the Midwest with rust early enough in the season to require fungicide application,we pretty much go along with the guidelines the folks in the South have developed,” says Shaner.

Meanwhile, Clements sees early maturing beans as a way to help offset early rust invasions. He grows soybeans, corn, wheat, sorghum and rice near Greenville, MS, in the Delta. Beans are planted either in 38-in. twin-rows with 7.5-in. splits or in straight 38-in. rows.

“Soybeans are now about 70% of our production,” says Clements, who farms with Ted Smith and Caroline Hicks. “We plant mid- to late Group IVs and very early Group Vs.

“We've seen rust get closer to us, but still haven't seen it until later in the fall. With the earlier maturing varieties, we see a big advantage in preventing rust from hitting our beans at the peak time,” he says.

Rust spread into portions of the Delta and other southern regions by late summer or early fall in 2007. It was part of the fungus invasion that found rust in 335 counties in the continental U.S. through Dec. 31, the highest number of counties reporting the disease since it was first discovered here in 2004.

Soybeans are most threatened if rust occurs during the R1 flowering to R6 full-seed stage. Andy Moore,a crop consultant and rep for UAP in Greenwood, MS, says planting Group IVs should help growers get past the danger stage.

“There's no reason why planting an earlier variety that reaches maturity quickly may help save growers from at least one fungicide application for rust,” Moore says.

That's a savings of about $10/acre for the fungicide, plus about $5/acre for the cost of aerial treatment application, he says.

FOR EARLY GROUP IVS planted April 10, plants normally reach the R6 stage about July 23 in the Delta area — before rust has been apparent in the region in recent years.

In 2007, USDA's Asian soybean rust monitoring site ( showed that rust was first confirmed in the Delta area about Sept. 15. Moore says research at the MSU Stoneville research center indicates that the April 10-20 planting period should be ideal for Group IVs, which would reach full maturity between Aug. 15 and 20.

Clements completes most of his harvesting in August, but some does go into September. “With the earlier maturing varieties, we can get beans in sooner and see a quick stand. We also see pollination earlier and miss the worst heat (usually) in July and August. We have the harvest advantages, plus get our beans before the rust-damaging stage,” he says.

GETTING BEANS OUT of the field earlier is an advantage southern growers can have over the Midwest, says Purdue's Shaner. So knowing when and what fungicides to apply can be critical if rust goes on a rampage.

SSRP, headquartered in Princeton, KY, has funded fungicide standardized trials to control Asian soybean rust over the past two growing sessions. Its trial results confirm that the first application must be made at or before rust infection begins on the lower leaves.

Based on the weather, the second application should be 14-21 days later. The canopy must be penetrated to ensure good spray coverage. Again, based on SSRP's 2007 study, the first application should be made between the R1 and R3 stages and should be a strobilurin fungicide if the rust risk is only moderate. If the risk is high, then a triazole fungicide must be used, or a combination of triazole and strobilurin.

Strobilurins are more effective on other late-season diseases, such as frogeye leaf spot, brown spot and anthracnose, says SSRP. If the threat of rust still exists, the study recommends that the second application, made between stages R3 and R5, be a triazole or one of the tebuconazoles — or a combination of triazole and strobilurin.

Shaner says growers should check for labeling before using these fungicides, as some may not have a current Section 18 approval. He points out that most fungicide labels specify that applications be made prior to the R6 growth stage, according to recommendations from an Ohio State University fungicide guideline (see chart).

“The only real difference between the Midwest and the South, to the best of our knowledge, is that rust will start earlier in southern states because that is where the fungus overwinters,” says Shaner. “The rust needs to build up there to generate enough spores to provide inoculum for the Midwest.”

SSRP (and others in the South) recommend strobilurin if risk is moderate and there is no rust actually in a field, because this class of fungicide does well against some other foliar diseases that can be a problem. The SSRP report notes that if rust is present at low incidence, a triazole is recommended because this class of fungicide has better curative activity.

Shaner says one big uncertainty for Midwest soybeans is whether the disease would need to build up on beans in the South. “There's a lot of kudzu down there, and if the disease built up on that host during the spring, spores could move from kudzu into the Midwest not much later than they reach soybean fields in the South.

“This scenario would require a storm that moved from the Gulf into the Midwest. More likely, though, growers in the South would need to treat fields at an earlier stage of development than would Midwest growers.”

He points out another difference between the South and Midwest is other foliar diseases. “In general,foliar diseases are not a big issue up here,” he says. “We usually have brown spot, but this stays down on the lower leaves until soybeans are maturing, and we don't think it does much damage.”

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