Soybeans were likely planted later than ever this year, and over a large geographic area in the Upper Southeast.

And North Carolina State University Plant Pathologist Steve Koenning says some of these beans may be exposed for longer periods of time to more than usual disease pressure.

In North Carolina, where a soybean crop of more than 1.5 million acres was predicted, many growers were still trying to get wheat out of fields targeted for double-crop beans late into July.

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However, some conventional beans planted back in May are near maturity, providing a long period of time for soybeans to be exposed to a number of pest issues.

As of the end of July, soybeans emerging and blooming were running 20-30 percent behind the five-year average for the crop.

The North Carolina State specialist, speaking at the recent Northeast Ag Expo in Shawboro, N.C., says the big threat is Asian Soybean Rust. He stresses ‘threat’ because the disease that annually ravages soybeans in South America has never really been a production problem in the U.S.

Koenning says the buildup of inoculum farther north than in past years is reason for concern. Combine that reality with what appears to be a very late maturing crop, and there is plenty of reason to be concerned about Asian Soybean Rust this year.

North Carolina State Soybean Specialist Jim Dunphy says the recent confirmation of Asiatic soybean rust on soybeans in a sentinel plot in Lee County, Ala., on July 28, puts rust a little closer to North Carolina soybeans.

However, he says, don’t panic and spray for rust, if you don’t have to do so. Still, rust, as of the last week in July, was not close enough to warrant a recommendation to spray for the disease.

Asian Soybean Rust was most recently detected in a soybean sentinel plot at Auburn University in Lee County, in east-central Alabama on July 28.

The disease was also reported on kudzu in Brooks County, Ga., on July 26, and in Hancock and Harrison counties in Mississippi on July 24.

On July 25, the disease was found in a soybean sentinel plot in Washington County in the northwest Florida Panhandle.

Closest confirmed source

The closest confirmed rust on soybeans to North Carolina soybeans in late July was approximately 310 miles from Charlotte, 575 miles from Elizabeth City, 400 miles from Fayetteville, 190 miles from Murphy, 440 miles from Raleigh, 515 miles from Washington, 440 miles from Wilmington, and 380 miles from Winston-Salem, N.C.

“We do not recommend spraying soybeans that have not started blooming with a fungicide to control Asian soybean rust. Such pre-bloom applications have seldom improved yields.  

“Once soybeans start blooming, we would recommend spraying if rust has been confirmed within 100 miles of the field,” Dunphy says.

Of more immediate concern, Koenning says, is frogeye leaf spot, caused by Cercospora sojina. 

“Growers need to scout their beans for this disease, and if they find it, they need to spray a strobilurin fungicide as timely as they can,” Koenning says.

He adds that long-term management of frogeye leaf spot is difficult because growers can go through several years when the disease isn’t a problem.

Growers may not have a problem with the disease for several years, so new varieties come out and have really good production characteristics, but may lack the same high level of resistance to frogeye leaf spot. “In the interim, growers tend to move toward these newer varieties, and when we get a year with significant pressure from the disease, we have more soybeans that are susceptible, he says.

Despite the changes in soybean varieties, Koenning says, “Most soybean varieties currently grown are resistant to this disease, and the use of resistant varieties is the preferred method of control.”

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Although frogeye leaf spot is seed borne, it tends to be worse in fields of continuous soybeans. Only newly formed leaves are susceptible to this disease, and fully expanded leaves are resistant until they start to senesce. 

Immature leaves become infected with periods of rain or high humidity, but infection will be limited by dry weather.

So, as the soybean plants put on new layers of leaves, frogeye may be present or absent depending on weather conditions during leaf expansion. This can lead to a situation where frogeye is layered in the canopy at different levels. 

Frogeye could be significant problem

If the wet weather that forced many growers to plant soybeans later than planned continues, frogeye leaf spot may be a significant problem this year, Koenning adds.

Frogeye has caused yield losses of 30 percent in some fields, so the general recommendation for susceptible varieties is the application of a strobilurin type fungicide, especially if continued wet and/or humid weather is expected. 

“We do not have a threshold for number of spots or percent leaf area affected to justify fungicide application. 

“If wet and/or humid weather persists as plants start to senesce, older leaves become susceptible again, and the plant may defoliate early,” Koenning says. 

“Early defoliation can result in smaller seeds which will translate into yield loss.  Also, pod infection can cause a reduction in seed quality or contribute to seed rot,” he adds.

Another tricky disease that many growers in the Upper Southeast will likely deal with this year is cercospora leaf blight. Whether it is actually a disease or an endophyte needed to help fight insects is not entirely clear, Koenning says.

“It’s hard to get a handle on cercospora,” Koenning says.

In some states, like Louisiana, researchers contend it is their No. 1 disease in soybeans. Closer to home Virginia Tech Plant Pathologist Pat Phipps says it may be Virginia’s No. 1 disease pest of soybeans.

Cercospora leaf blight is caused by a pathogen closely related to a pathogen that causes frogeye leaf spot. Cercospora leaf blight of soybeans is caused by the fungus Cercospora kukuchii, a close relative of Cercospora sojina that causes frogeye.

This late-season leaf and seed disease shows up in during pod-filling stages. It is relatively easy to identify by a light purple to bronze discoloration that resembles a sunburn on the uppermost leaves.

The coloration deepens and takes on a leathery appearance as the plants approach maturity.

Seeds can also be infected, resulting in a seed disease called purple seed stain. If infected seeds are planted, the fungus grows through the seed coat to the cotyledons and into the stem.

The key to managing cersospora blight in soybeans is to detect the disease very early, Koenning says. If growers wait until the R3 stage or later to spray, fungicides typically won’t keep the disease in check, he adds.

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