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Illinois Corn Update: Ear Molds, Weeds & Yields

October 24, 2016

ILLINOIS: The past three months saw above average temperatures and rainfall which usually has an effect on the corn crop. Experts are seeing lower than projected yields, ear molds and more weeds than expected.

Here are the numbers according to the National Weather Service in St. Louis:


  • Rainfall was 8.37” total which is 4.26” above normal
  • Low temperature was 2.1 degrees above normal at 73.1 degrees
  • High temperature was .9 degrees above normal at 90.0 degrees
  • July 2016 was the fourth wettest on record and had 28 days where the low temperature was above 70 degrees


  • Rainfall was 5.77” which is 2.78” above normal
  • Low temperature was 3.1 degrees above normal at 72.5 degrees
  • High temperature was 1 degree above normal at 88.9 degrees
  • August 2016 was the 11th wettest on record and is tied with the 20th warmest on record


  • Rainfall so far has been 4.82” which is 2.22” above normal
  • Many areas have seen much more rain locally
  • The month finished with both high and low temperatures above normal, as well

So what do these numbers mean to a corn crop?

  1. Above average night time temperatures are associated with higher respiration rates in the corn plants. Respiration uses sugars formed from photosynthesis during the day, leaving less sugars and energy available for grain fill. Additionally, higher temperatures speed up corn maturation leaving less time for grain fill. Cooler temperatures, especially nighttime temperatures, result in a longer grain fill period, less energy loss and higher yields.
  1. Ear molds are always more likely to occur as a result of above average moisture during the grain fill process. During periods of high rainfall and humidity, heavy overnight dews are common and ear molds have excess moisture available for continued development. Insect feeding on the ear tips can worsen this condition. Sprouting of kernels is a common sight as well this year. More information about Diplodia ear mold can be found here.
  1. Ample moisture and warm temperatures moved the corn crop along quickly. Nitrogen loss was also a factor in some fields. A severe infestation of Southern Rust was also present. Similar symptoms of nitrogen deficiency, disease and crop maturity caused green leaf tissue to disappear in August, more sunlight broke through the crop canopy reaching the soil.

Because of excessive moisture conditions, weeds were able to emerge giving fields a “weedy” appearance after harvest. Two key points to consider are how long the herbicides had been out in the field and the continual breakdown of those herbicides that occurred due to the above average rainfall and temperatures throughout the last 90 days.

Usually plenty of moisture and warm temperatures are a good thing for corn crops, however during the summer of 2016, there were no real breaks in the weather pattern to offer any relief to the crop which forced it to use up more energy than normal in overnight recovery. Constant high moisture allowed ear molds to set in and cause damage and herbicides were tested to the extreme throughout the growing season.

Submitted by Phil Krieg and Nate Prater, agronomic service representatives for Syngenta, and Doug Kirkbride product development agronomist for Syngenta.

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