Stresses during corn reproductive stages, late season nitrogen management considerations, and corn disease considerations

By: Ryan Miller, University of Minnesota Extension Educator - Crops, Jeff Coulter, University of Minnesota Extension Corn Agronomist, Brad Carlson, University of Minnesota Extension Educator - Nutrient Management and Water Quality, and Dean Malvick, University of Minnesota Extension Pathologist - Field Crops

Environmental stresses

The “heat dome”, hot temperatures during the early reproductive stages of corn development, what does it mean for Minnesota’s corn crop? The Minnesota corn crop is about a week ahead of normal and currently has adequate moisture but limited precipitation or chances for precipitation means things are drying out. Fortunately, the temperature conditions are expected to be cooler this coming week.

So how is the heat affecting corn that is pollinating or starting to pollinate? Current high temperatures have been in the low 90’s, and corn can tolerate those temperatures without issue. Things have been drying out with the lack of precipitation and during hot dry conditions we can see tasseling speed-up and silking slow down a bit, so some of the later emerging silks might not get pollinated under stressful conditions. Air temperatures in the mid-90s or above reduce pollen viability, but that is generally not a concern because most pollen shed occurs in the morning hours while temperatures are cooler. Cooler temperatures next week will benefit Minnesota’s corn crop as more corn starts to pollinate and corn that has been pollinated enters a critical phase of kernel retention.

A few days after pollination has started you may want to check if you had successful pollination. Silks grow about an inch per day and silks that have not been pollinated will continue to grow longer. To check pollination growers can carefully un-husk an ear and gently shake the cob to see if silks detach, silks that detach from the ear indicate successful pollination and silks remaining attached indicate they have not been pollinated. With modern hybrids, tassel emergence and silking tend to be quite synchronous, tassels will continue to drop pollen for several days after they start pollen release.

Ranking the critical periods for environmental stress and corn yield potential. The most important is the week of pollination (10 of 10), then the two weeks after silking (9 of 10), and then three to four weeks after silking (8 of 10). During this period we can see stress affect pollination and kernel retention.

Nitrogen issues

With the dry start to the season the crop generated deep roots giving the plants access to water and nutrients across the soil profile. Fertilizers allowed for very even corn growth during the rapid growth phase of corn and currently the corn crop is showing no signs of nitrogen deficiency. Due to some of the recent heavy rains and saturated soil conditions there was some concern with the potential for denitrification and nitrogen loss in several counties in Southeast Minnesota. Rest assured that with the adequate soil moisture and warm temperatures our soils with 2.5% or greater organic matter have great potential for mineralization and release of nitrogen, making up for any losses from saturated soil conditions.

Foliar Diseases

Which foliar diseases have been showing up on corn in MN so far this season?

• Tar spot has been found and reported at very low severity levels in several counties in Southwest Minnesota and in one county in Southeast Minnesota as of 7/10/26. See the corn IPM PIPE web site for counties.
• Northern Corn Leaf Blight has been reported in Southeast and Southcentral Minnesota at very low levels.
• Goss's leaf blight and wilt has been reported in Southwest Minnesota

What is the risk of southern rust developing this year?

• As of July 17th, southern corn rust in the U.S. has only been reported inTexas, Mississippi and Missouri so far, indicating this disease is spreading and developing much more slowly than last year. This may suggest that the risk of this disease is lower for MN this year, but we don't know the risk or the dispersal patterns for the southern rust spores. We will learn much more in the next few weeks.

Disease pressure has been low in 2026 and we will need to continue scouting to monitor for disease development. If we find evidence of disease we can consider our management options.

Tar Spot

If we find multiple lesions on a few leaves and the extended forecast is for moderate air temperatures and humidity, and occasional rainfall, we may consider the use of a foliar fungicide for control. The crop disease risk tool on the crop protection network website can help assess the risk of corn tar spot.

Southern Rust

Continued scouting will be necessary to see if this disease arrives in Minnesota in 2026. Management of this disease will depend on if and when southern rust arrives. If the disease arrives later in the season we will not need to consider management. Management with fungicides typically occurs between the VT and R3 stages of corn, but the disease needs to be present and conditions must be correct for disease development.

Goss’s Wilt

Scouting for this disease is important. If goss’s wilt is found it can not be managed with a fungicide. Hybrid selection in future crop years will be important because resistant hybrids can be used to manage this disease.

Northern Corn Leaf Blight

Northern corn leaf blight develops best in cool and wet weather, so weather conditions would need to change significantly for this disease to become a problem.