off-colored soybean leaves
MARKS APPLICATION: These off-color leaves were likely caused by a herbicide application made to take out Canada thistle. Because of good growth afterward, Steve Gauck expects no yield effect.

Distinguish between herbicide burn, disease

Soybean Watch: Neither may hurt bean yields, but it’s good to monitor what happens in the field.

Sometimes you drive by a field and everything looks OK. You don’t think there is anything you can learn by parking the pickup and walking out into your crop.

“One thing the Soybean Watch ’17 project has reminded me is that there is always something to learn in the field,” says Steve Gauck, a sales agronomist for Beck’s, based near Greensburg, Ind.

“Often I get called to look at a specific problem in a field. I don’t get to walk fields very often just to see how they are doing,” he notes. “We found out this year that even if the field looks normal from the outside, there are stories to find and lessons to learn if you walk inside. What you find may not always be a problem that will hurt yield. But it’s often something you can learn a lesson from and apply the next year.”

Soybean Watch ’17 is sponsored by Beck’s.

Scouting in August while this year’s field was in the middle of its reproductive phase, Gauck learned some of those lessons. The field was planted June 6 after weather delays. Wet weather also delayed spraying. Canada thistles got a good start and were poking above the canopy before they could be sprayed. Gauck visited the field a couple of weeks after the application was finally made.

Determine spraying time
“We noticed a few discolored leaves on several plants,” Gauck reports. “They tended to be about 6 inches below the top of the canopy. If you knelt down and looked closely down the two rows, they kind of lined up, indicating what stage the beans were in when they were sprayed.”

Gauck suspected the farmer added something besides glyphosate since leaves in the middle of the canopy showed those symptoms. Sure enough, the farmer reported that he had added Cadet to the mix.

“That’s why there was some leaf burning,” Gauck says. “But it’s probably also why he smoked the thistles. He did a very good job of killing the top growth of thistles.”

As for the burned leaves, Gauck isn’t concerned. “From what we’ve seen before, I wouldn’t expect any yield loss because of it,” he says. “The fact that plants have put on new sets of leaves after that, and are still blooming and setting pods is a good sign.”

Damage vs. disease
A few of the leaves that had a similar appearance to the leaves showing herbicide burn also had a rough, bumpy surface. Gauck pulled out the Purdue University Corn & Soybean Field Guide that he carries with him while walking fields.

He soon identified it as cercospora leaf blight and purple seed stain. “The bubbly, leathery appearance on the upper leaf distinguishes it from leaves which showed signs of herbicide burn,” he says. “It typically shows up more when there is higher humidity and heavy dews. We have definitely experienced some of that type of weather lately.”

Gauck doesn’t expect the disease will harm yield either. It may produce a few beans with purple seed stain.

BURN VS. DISEASE: Note the rather smooth appearance on the left leaf vs. the bumpy surface on the right. Steve Gauck says the left leaf was likely damaged by herbicide, while the right one is diseased.
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