Widespread reports in July from farmers across Minnesota about soybean leaves cupping prompted University of Minnesota Extension scientists to host a special crops podcast to discuss why plants were looking this way.

Comments heard from across the state expressed concern and conjecture, linking the symptom to dicamba drift, herbicide residual carryover, susceptible soybean varieties and/or drought stress.

The bottom line: No one really knows why soybean leaves are cupping.

The July 16 podcast featured U-M Extension weed management specialists Debalin Sarangi and Tom Peters; Integrated Pest Management specialist Bruce Potter; and Extension soybean agronomist Seth Naeve.

“There are a lot of variables this year,” noted moderator Jared Goplan, an Extension crops educator based in Morris. “I’ve had a lot of interesting discussions with folks.” He listed several reasons that could cause leaves to express cupping: incredibly hot weather, insect and/or disease stress, herbicide carryover (from applications used to kill thistles), spray-tank-boom cleanout, herbicide drift, herbicide application response during dry conditions, atmospheric loading and growth regulators.

“That’s a lot of things that could be going on in soybean fields,” he said, adding that this podcast’s goal was to discuss observations and possible reasons, not to point fingers.

From a plant physiology standpoint, Naeve said the soybean plant is still vigorously growing vegetatively and is not investing much in flowers and pods. In essence, it’s doing exactly what it should be at this stage of growth.

Peters said he believes there is a subtle malformation in phenotype taking place and Naeve agreed.

“Tom hit on a good point,” Naeve said. “Saying ‘phenotype’ is a good descriptor of the genetics and environment together. It’s a combination of the two. We’re seeing this phenotype that is questionable. There is not a heavy growth effect on the plant. As long as there is continued growth and leaves, we won’t see anything from it. ... It’s important for people to understand that it’s a broad-based phenotype that won’t have an effect on yield at all.”

Naeve added that specific to cupping, a plant growth spurt caused by warm weather and/or a small amount of rain could cause weird expression in leaves.

From a possible chemical impact standpoint, Sarangi explained that different plant symptomology is caused by different herbicides. For example, dicamba would show leaf cupping, while 2,4-D would cause banana-shaped cupping. Group 15 herbicide causes leaf crinkling.

“The only Liberty injury I’ve seen so far is a little yellowing in the leaf margins from spraying, and then that usually goes away,” Sarangi said. He also agreed with the others that multiple factors are probably causing soybean leaf cupping.

A good resource to see the difference between 2,4-D and dicamba injury is available at this Purdue University webpage on 2,4-D and dicamba injury in soybeans , he said.

Peters took issue with the possibility of tank contamination as a possible cause, because cupping is uniform across impacted fields.

“If it was tank contamination, you would see a pattern,” he said. “And if drift was occurring, we would see a feathering effect, where it would be heavier on the edge and lighter as you go across.”

Regarding comments heard about tolerance differences in dicamba soybeans, Naeve said there is some variation in sensitivity. That is to be expected since early varieties usually come with more narrow germplasm.

Herbicide volatility

When farmers were spraying herbicides during peak application time in early to mid-June, it was very hot and dry across most of the state. So where were those herbicide molecules going? Goplan asked.

“Non-dicamba soybeans are the canary in the coal mine,” he said. “We were struggling to have good application conditions this year. The heat wave made that challenging.”

The others agreed that volatility and atmospheric load is a concern.

“Atmospheric load can blow a couple miles and pose injury,” Sarangi said. “We can’t ignore temperature inversion either.”

Peters noted there has been potential for herbicide inversion, given the weather conditions. He estimated that conditions were conducive for inversions three days per week for the last three weeks.

“We know higher temps lead to more volatility,” he added. “We learned that in the 1960s, that they could be coming back from miles from where they were applied.”

More than once, the Extension specialists noted that more research is needed to help explain why plants are responding this way.

“We have gotten ourselves in a box with herbicide technology regarding adjacent soybeans with incompatible herbicide tolerances,” Potter said. He noted that in the past, there has been dicamba drift, herbicide carryover, off-label dicamba applications and use of unapproved tankmixes.

“But these are not going to explain everything,” Potter said. “We do not want to get in a position of accusing people of things without some actual knowledge of what is happening. … There is more going on than dicamba injury. Hopefully, science will help define [what is going on].”

Goplan said it was important for growers to keep records of what they did and what they are seeing. Data collection will help researchers in future projects.

“Approach your fields objectively,” he said,” and have an open eye.” If you see evidence of off-target herbicide applications, he suggested filing complaints with the Minnesota Department of Agriculture on this MDA webpage.