Group 4 soybeans hurt late

Early-maturing Group 4 soybeans planted in Arkansas the second to third week of May are having problems. The third week of August saw some plants shutting down about two weeks prematurely.

When the soybeans began drying down, “many growers thought the situation was natural,” says Scott Monfort, Arkansas Extension plant pathologist. “It wasn’t until consultants and county Extension agents began checking closely that they found beans weren’t filled out and shouldn’t be drying. The tops of the plants didn’t have anything but BBs in pods.”

Some situations “are pretty bad and farmers could see an estimated 10 to 15 percent reduction in their yields. That’s not a certainty, but it looks that way, right now.

“It’s hard to say precisely how badly yields will be hurt. I’ve looked at some of this problem in Prairie County (in central Arkansas) and it doesn’t seem as bad as it is farther north. The pods there are already full to the top although they’re ‘sitting down’ earlier than they should.”

What’s being seen in the soybeans is likely a complex of factors, not a single problem. Early in the season, from late April into May, soybeans received quite a bit of rain. That allowed the plants to expend energy on things other than root systems. Since water was near the surface they didn’t need to search deeper for moisture.

Then, at the beginning of July, as the soybeans reached reproductive stage at R-2 (a sensitive time for soybeans), the rains tailed off. The crop wasn’t equipped to handle the moisture shut-off.

“I believe we had stress on both extremes: too much water up front and not enough during the critical bloom stage. That led to some root damage. Even though it wasn’t severe damage, it was enough to hurt the crop.”

Without water, root tips will burn or dry out and plants will begin to decline.

“A lot of diseases beginning to pop up normally don’t cause us widespread problems. Since the root systems were so stunted, even short times between irrigations allowed the plants to dry out. That stressed the plants during pod-fill.”

The plants had been so stressed that when irrigated, “they were shocked. That made many of these problems worse. That’s certainly different than what we were expecting.”

In some of the worst hit areas, “by the time the crop was filling pods, the plants were simply unable to keep up and they shut down. That’s why the crop is drying out so early.

“When the plants got stressed so badly some pathogens moved in and started working on the crop. Those include some Fusarium, charcoal rot and that sort of thing.”

Asked if the range of symptoms in the crop has been seen before, Monfort says the range of stresses may be the better place to start an investigation.

“I’m new in the state, but I understand Group 4s haven’t been grown in Arkansas widely until the last five to 10 years. Group 4s don’t seem to do as well as Group 5s under these extreme stress conditions.

“I suspect this type of thing normally happens in pockets and doesn’t raise eyebrows. But the thing is, since 1980 we haven’t had four straight weeks of 100-plus degree temperatures. This is not a typical season — once every 25 years, or so.”

At the same time, both Mississippi and Louisiana have reported odd maladies in their soybean crops.

“Cliff Coker (Arkansas Extension plant pathologist) traveled to Mississippi to check some of the mystery disease problems in their soybeans. He diagnosed a lot of it as charcoal rot.

“And, to some degree, that’s what we’re seeing here in Arkansas. Charcoal rot is definitely in the fields. But the extreme drought and heat in August are the chief culprits here, I think.”

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