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Disease spread in Southeast: Soybeans race rust for finish line

With Asian soybean rust nipping at its heels, the Southeast soybean crop is nearing the finish line. As pods fill and rust-ravaged leaves fall, R-6 designation — and safety — can't come soon enough for producers.

“Except for the extreme northwest and northeast corners of the state, we're picking up rust in all our sentinel plots,” said Bob Kemerait, Georgia Extension plant pathologist on Sept. 9. “We're also picking up rust in commercial fields. We're in a race with rust now. Where we have commercial fields in my fungicide trials, the beans are either at, or approaching, full seed. Our understanding is once they reach that level of maturity the crop is fairly safe from rust.”

The first two weeks of September saw more discoveries of the disease in Alabama, Georgia, Florida and South Carolina.

Soybean rust remains a threat in Alabama but may be slowing. After a spate of findings in August, the state has announced just one new case in the last two weeks: a Chilton County sentinel plot.

“That plot is just north of Montgomery,” said Ed Sikora, Alabama Extension plant pathologist on Sept. 9. “I'm really pleased we haven't found more. The problem with saying we're out of the woods is Alabama's beans aren't consistent in maturity. We have a lot of double-crop beans scattered around. I was in north Alabama just prior to the hurricane and they were already harvesting Group 3s and 4s. So it's true that a good deal of the crop is being harvested or is nearing it. But we have quite a few double-crop beans still at risk.”

In Georgia, where yields look “very good,” September findings have been in Spalding, Tatnall, Tift, Washington, Laurens and Marion counties.

“By now, most of our growers are very aware of what they're facing with this disease,” said Kemerait. “I believe the critical thing now is with growers who are still in early pod-fill stages. With the plentiful amount of rust now in the state there's reason for them to be worried.”

The big question for growers who haven't sprayed a fungicide recently (or ever) is: “Will beans reach maturity before rust-caused defoliation nails them? Is the rust coming in so late that, even though it's present in the crop, it can damage yield? We'll find out.”

Defoliation is occurring in some Georgia fields, but Kemerait is happy it isn't at the level feared prior to the season's start. “Information out of Brazil indicated there would be massive defoliation within two weeks of initial infection. That hasn't happened here. Rust just isn't moving as quickly here as it has in Brazil. But it's an impressive disease once it gets established.”

Kemerait “knows” soybean rust is more widespread in Georgia's commercial fields than has been reported. “However, it still isn't at the point where there are huge areas of complete defoliation. At the most advanced stage of the disease, we're seeing the lower canopy leaves having fallen off with the mid-canopy now being affected.”

Currently checking soybeans around Mobile in Baldwin County, Sikora will head the opposite direction next week. “On (Sept. 12) I'll be in north Alabama on the Tennessee border looking at more fields. If there's any spread of the rust due to Hurricane Katrina, we should begin seeing symptoms about then.” It takes about two weeks after spore deposits for rust symptoms to show up.

“I'm very interested to see if Katrina pushed rust into some of our northern sentinel plots and commercial fields.”

As the R-6 nears for much of the crop, scouting efforts in Alabama haven't slowed. “I think we'll be looking for rust through November. We'll be checking volunteer beans through the fall. We want to track it as closely as we can.”

Back in Georgia, Kemerait said many remain slightly intimidated with identifying the disease. “In the Laurens County field report, we said rust was fairly advanced and yet wasn't recognized. That has made a lot of growers nervous and spurred more calls to us suddenly.

“I believe once you see this disease, you get comfortable identifying it. But many growers are a little tentative to make the initial identification. Even some of our Extension agents are that way. The first time they see it, they want to line up assurances that's what they're seeing. After the first positive ID, though, they'll make the second call on their own.”

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