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Researchers explore early signs of new soybean disease

Researchers explore early signs of new soybean disease

soybean affected by red crown rot

Red “balls” on the lower crown of a soybean affected by red crown rot. Note the blue growth- that’s the causal fungus of Sudden Death Syndrome. Remember that co-infection can occur, so don’t assume anything is RCR or SDS without checking those stems and roots.

A new disease culprit is threatening to rob soybean growers of some yields, although there has been relatively little damage so far.

Red crown rot, caused by a soil-borne pathogen, was spotted in some fields in 2020. It was first identified in 2017 in Pike County, Illinois, with a few incidences turning up the following year, according to the University of Illinois.

The disease develops from a fungus producing a toxin that moves up through the stems into the foliage of soybean plants. It accumulates in the leaves, killing the tissues before leaving over time. Severely affected plants die or senesce prematurely, similar to sudden death syndrome, which has been present in the state for years.

“It’s an infection that starts at planting,” said Ben Weigmann, an agronomist with Beck’s Hybrids. “… Cool temperatures can lead to some of the infection.”

Weigmann said he has seen little of the disease in the southern Illinois region where he is based.

“The leaves will resemble SDS, giving the plant an interveinal sclerotic look,” he said. “If you slice the stem open, the pith will be a reddish color. It will typically be around the same timing as SDS also. It’s similar to the infection in that moisture induces it sometimes. That’s a primary leading factor.”

Craig Kilby, an agronomist who works in the western Illinois area, is also aware of the disease. Like Weigmann, he has not gotten many reports of it.

University of Illinois researchers are looking into the disease. Led by pathologist Nathan Kleczewski, the team is working with farmers on identifying it in fields around the state.

If conditions have been wet, they may see small round “balls” on the lower stem of the plant. It may also be covered by a white fungal growth.

“The key here is the presence of fungal tissues, not simply a reddish coloration to the stem,” Kleczewski reported in a paper.

Several factors can cause a reddish tint to lower stems, he pointed out, so that is not necessarily an indication of the presence of red crown rot.

The suspect plant can be carefully removed from the ground with a shovel, with the soil gently knocked off the roots.

The university is working to better understand the distribution and management of the disease in Illinois.

Weigmann said fungicides would have no effect on the disease, since it is soil borne. He recommends that farmers concerned with reports of it near their farms continue to plant soybean varieties resistant to sudden death syndrome.

“Seed treatments can help a little bit with it. It can limit the infection at the beginning of the season,” he said. “Planting date could be something big to get out of those cool, wet conditions. When we have a soil-borne pathogen like this, a foliar fungicide won’t have any action because it’s coming up from the soil.”

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Nat Williams is Southern Illinois field editor, writing for Illinois Farmer Today, Iowa Farmer Today and Missouri Farmer Today.

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