Injury symptoms on corn plants. While passing by on the way to plant a bulk soybean field on Tuesday, personnel at the Northwestern Illinois Agricultural Research and Demonstration Center (NWIARDC) noticed that plants in one of the center's corn fields didn't look so hot (Figures). The crop was planted on April 16 and sprayed on April 25 before emergence.

The products that were sprayed included 2.5 qt/A of Lumax (atrazine + s-metolachlor + mesotrione) and 16 oz/A of 2,4-D. Both products are labeled for either pre- or post-emergence application in field corn and the rates are well within label guidelines.

Individual plants had injury symptoms that ranged from small, water-soaked or necrotic leaf lesions to no living above-ground tissue (Figures). In corn treated with this same combination of products at the same rates in 2015, no such injury symptoms occurred. What was it about the two growing seasons that differed?

Weather conditions. Weather that exposes plants to cool, wet and cloudy conditions does not favor growth and development but rather stresses plants. While corn is typically able to metabolize and survive exposure to selective herbicides, injury may occur when metabolism is significantly slowed. Metabolic processes are slowed when plants are exposed to unseasonable cool and cloudy days. In 2016 plants experienced more days with low quantities of total solar radiation than they did during the same period in 2015 (Figure). Average daily temperatures were also higher in 2015 than they were in 2016 in 15 of the 24 days after the herbicides were applied (Figure).

The good news. While these corn plants were obviously very severely injured in 2016, the growing point, which remained below ground, appeared to be unaffected. Additionally, as the seed was not planted abnormally deep (1.75 inches) and the seedlings had not yet used all of the energy reserves in the seed (check by squeezing the seed) the plants are likely to regrow.

However, one must remain aware that seedlings essentially restarting their above-ground growth may continue to remain behind unaffected plants maturity-wise and yield potential may be reduced. In recent years very seldom has injury been both severe and widespread enough for farmers, particularly those with crop insurance, to consider replanting. The warmer temperatures forecast for the next week are likely to speed crop growth and recovery.

References

Water and Atmospheric Resources Monitoring Program. Illinois Climate Network. (2014). Illinois State Water Survey, 2204 Griffith Drive, Champaign, IL 61820-7495.

Additional Resources

Herbicide Injury Symptoms on Corn and Soybeans. Department of Botany and Plant Pathology. Purdue University.

Practical Weed Science for the Field Scout: Corn and Soybean. University of Missouri Extension.

Herbicide Mode of Action Key for Injury Symptoms. University of Wisconsin Corn Agronomy.

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