Begin Planning Final Irrigation for Corn, Soybeans
Published: Friday, September 11, 2026
This article is based on Sept. 3 data.
The past week was mostly warm and dry, with some showers over the weekend. High heat and humidity early this week were a big contrast to the fall-like weather from the previous week. Thunderstorms today (Sept. 3) leading into showers tomorrow are expected to bring about a third of an inch of precipitation. The weekend is forecast to be dry and warm, with the next rainfall expected early next week.
Full season soybeans are finishing R5 (seed fill) and entering R6 (full seed). Several fields have started to senesce. Once one pod on the stem reaches its mature color, the soybean is at R7. Continue to watch out for red crown rot, which is a potential disease not yet confirmed in Michigan. Symptoms look similar to sudden death syndrome and brown stem rot.
If you suspect red crown rot, submit a sample to the Michigan State University Plant and Pest Diagnostics Lab. The Michigan Soybean Committee and the United Soybean Board will cover the diagnostic fees. You can also request a soybean sampling kit.
Most corn fields have entered R5 (dent), and some are at R6 (black layer). At R5, the milk line is visible. This is a change in color in the middle of the kernel that shows where solid starch accumulation (darker color) has occurred, starting from the outer edge and moving inward. At this point in the season, scout for ear molds and stalk rots, which can affect harvestability and seed quality. Seed corn harvest began this past week.
Potato harvest is picking up and will begin in earnest in a couple weeks. Many fields have already been terminated to prepare for harvest, and the rest will follow within a week or so. There have not been any further late blight detections since early August when it was found in St. Joseph County. Potato field after desiccation.
At R5 stage, corn still uses approximately 0.7–1 inch of water per week. Once corn reaches the black layer (maturity), water use decreases to about 0.6 inches per week, and additional irrigation may no longer be beneficial. Start thinking about your last irrigation event. Once corn reaches black layer, there is no benefit to adding more water.
Soybeans at R5–R6 remain at peak water use, at approximately 1.16 inches per week in southwest Michigan. Before making your last irrigation decision, check soil moisture to ensure there is enough water available to carry the crop through full maturity. When approximately 50% or more of the leaves have turned yellow, this can be a good endpoint for irrigation. When irrigating soybeans, using higher application rates can reduce the frequency of leaf-wetting events, which may help reduce conditions favorable for white mold development.
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