Treating for soybean rust a field-by-field decision

Treating for soybean rust a field-by-field decision

• Ideal growing conditions right now constitute a double-edge sword Alabama for soybean producers. • What’s good for soybeans is also good for some major soybean diseases, not to mention, a few soybean pests.

In the midst of these ideal late-summer growing conditions, an Alabama Extension soybean agronomist says fungicide sprays may be warranted for beans that have reached the reproductive stage, but haven’t yet reached the R6 level, when seeds touch the pods.

Right now, thanks to ample rainfall throughout much of the state, late-planted soybeans have posted solid growth and offer great yield potential, says Dennis Delaney, an Alabama Cooperative Extension System agronomist.

Delaney shared this advice with growers during the annual East Central Alabama Crops Tour, held Wednesday, Aug. 22, with various stopovers throughout east and central Alabama.

But there is a flipside to this. As Delaney stresses, what’s good for soybeans is also good for some major soybean diseases, not to mention, a few soybean pests.

Researchers throughout the country enjoy what amounts to a bird’s eye view of this issue. They have developed what amounts to an agronomic canary in a coal mine — a series of what they call sentinel plots of soybeans that help them monitor for signs of one of the most virulent weather-related disease, soybean rust. 

The plots, planted at various times throughout the growing season, are designed to give Delaney and other soybean experts a clear picture of the diseases spread throughout the state.

“We want to get that continuous blooming — continuous green material that we can watch and sample as the rust moves north.”

As Delaney relates, soybean rusts follows a pattern: it overwinters on kudzu in the Gulf Coast — Baldwin and Mobile counties — and moves north, following the rain and maturing soybeans.

Becoming fairly widespread

He’s already noticed many soybean plant leaves throughout the state growing fuzzy with soybean rust, which has now been detected throughout central Alabama.

While the defoliation associated with rust is not a problem with early planted beans quickly reaching maturity, Delaney says it’s a major issue for late-planted beans in reproductive stages that have not yet reached the R6 stage development stage.

This leaves many growers with a dilemma: to spray or not spray.

“It always comes down to a question of one application or two,” Delaney says. “Most fungicides last for only about 3 weeks, and you really need to keep them covered for about 5 to 6 weeks.”

While the decision to spray — or, for that matter, not to spray — always boils down to a field-by-field decision, growers need to decide whether they can delay fungicide spraying or to apply it early, followed by a second application.

Much of it will depend on how healthy the beans look. However, Delaney says fungicide spraying is a good practice not only for rust but also for frogeye leaf spot and other diseases, for which cooler, wet weather provides ideal conditions.

Delaney says growers can usually average about a 3-bushel an acre improvement in yields when fungicides are applied, though the improvements could be markedly higher when ideal growing and disease conditions prevail.

Aside from rust and other foliar diseases, Delaney advised growers to keep a close eye on another foliar enemy, the kudzu bug, which preys on soybean foliage rather than pods. 

Though the insect is a form of stink bug, the damage they inflict on soybeans shares much in common with aphids.

“If you see a few here and there, don’t get excited,” Delaney says, adding that Alabama growers haven’t yet seen enough of the insects in fields “to get a handle on them.”

Much remains to be learned about the bugs, although Delaney says growers in Georgia are being advised to spray during their nymphal stages for optimal control.

For now, he says, the pests have not yet reached high enough numbers in Alabama to pose a serious threat.  Even so, he says this is no excuse for producers not to remain vigilant.


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