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Soybeans: scout for diseases

TAGS: Soybeans

Just when you’d think Cliff Coker has seen it all, Mother Nature throws a curveball. This year, the twist comes in the form of three soybean leaf diseases, despite a summer of dry, 100-degree days.

Typically, wet conditions like the August-to-November rain in 2009, promote growth of the usual suspects: anthracnose, frogeye leaf spot and aerial blight.

“It’s been an interesting summer,” Coker, an Extension plant pathologist for the University of Arkansas Division of Agriculture, said Monday. “I have thousands of plots around and I was surprised to see the frogeye leaf spot.

“I’ve actually recommended more fields be sprayed for our usual Arkansas foliar diseases than I have in four to five years,” he said.

Coker and fellow Extension plant pathologist Scott Monfort said soybean producers need to be scouting for fungi before considering a fungicide application.

“You can’t shoot from the hip and say, ‘I’ve always done it at R-3’,” Monfort said. “Without being in that crop and looking, you’re wasting your money.”

The reason for the disease development in some cases is surprisingly simple: irrigation.

“The well-managed fields are getting good timely irrigation and there’s plenty of moisture in the lower canopy,” said Monfort. “The upper canopy is blisteringly hot, but the lower canopy is high humidity.”

The humidity builds because closely spaced leaves prevent air circulation down below, creating a perfect place for spores to grow.

“A little bit of spore dispersal and a susceptible crop, you get a spot or two this week and the fungi produce thousands of spores,” Coker said. “And that little bitty spot becomes an opportunity” for a wider infection.

Coker said the appearance of these diseases is not a reason to stop irrigating. “You don’t get the yields if you stop irrigating.”

Producers who do spray for disease need to be sure there’s adequate volume to reach deep into the canopy.

“You can apply all the product you want, but if you can’t get it into the canopy, you gain nothing,” Coker said. “Spray according to what you see.”

Hank Chaney, Faulkner County Extension staff chair, said unique conditions created in the Arkansas River Valley also create a welcome environment for fungi to attack soybeans grown in the bottoms.

“The river valley tends to be (foggier) and keeps the moisture in,” he said. “If the grower doesn’t do a good job selecting a resistant variety, we tend to see a little more of the frogeye leaf spot.”

Joe Vestal, Lafayette County staff chair, said he’s seen frogeye leaf spot in a few fields.

Monfort said he’s heard reports of anthracnose and aerial blight in northeastern Arkansas, but the problem was not widespread.

For information on crop production, visit or contact your county Extension office.

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