Corn Disease Update 8-21-15

Corn Disease Update 8-21-15

 gray leaf spot symptoms on a corn leafNorthern corn leaf blight symptoms on a corn leaf
Recent weather conditions have been favorable for the continued development of corn gray leaf spot (Figure 1, left) and northern corn leaf blight (Figure 2). (Photos by Tamra Jackson-Ziems)

Recent Cooler Temperatures Favor Some Corn Diseases, Not Others

August 21, 2015

Gray Leaf Spot

Gray leaf spot (Figure 1a) continues to be a concern in many fields.  Some people are reporting increasing severity and development of new gray leaf spot lesions after foliar fungicide applications.

Learn how to identify and manage corn diseases at the Aug. 27 Crop Management Diagnostic Clinic near Mead. Among the speakers will be UNL Plant Pathologists Tamra Jackson-Ziems (corn) and Loren Giesler (soybean). Register today.

Keep in mind that there are a few scenarios possible in these fields.  A number of bacterial diseases are common in corn across the state.  Some of these diseases may look similar to other fungal diseases, like gray leaf spot. They will be unaffected by foliar fungicide applications and may still be increasing in severity.  In addition, it's important to remember that it can take 14-21 days for gray leaf spot lesions to develop so that they are recognizable.  Infections that took place several days prior to a fungicide application will continue to mature. 

Most of the commercially available fungicides for corn contain two components representing two classes and modes of action, often both a strobilurin and triazole.  Strobilurin fungicides protect uninfected leaf tissue from new infections and triazole fungicides can provide curative activity by stopping early infections that have just begun.  Treatments made during the dent (R5) substages are much less likely to provide a yield response than those made earlier.  Although, overall yield, especially test weight, could be affected by disease during the dent stage when as much as 55% of dry matter (Table 1) is left to accumulate in kernels (Abendroth et al., 2011).   

Table 1. Dry matter accumulation, grain moisture, and growing degree days (GDD) during dent (R5) substages.
Corn R Stage % Moisture* Dry Matter
(% of Total Dry Weight)
Avg GDD Avg Number of  Days
5.0 60% 45% 75 3
(1/4 milk line)
52% 65% 120 6
(1/2 milk line)
40% 90% 175 10
(3/4 milk line)
37% 97% 205 14
(Physiological maturity)
35% 100%    
    575 33
SOURCE: Abendroth, L.J., Elmore, R.W., Boyer, M. J., and Marlay, S. K.  2011.  Corn Growth and Development.  PMR 1009. Iowa State University Extension, Ames, Iowa
*Grain moisture and dry matter variation of at least +/- 2% is expected during R5 substages.


Northern Corn Leaf Blight

The fungus causing northern corn leaf blight (Figure 1b) is favored by the cooler temperatures that we are experiencing, especially in combination with higher humidity and dew.  This disease will likely also continue to increase in susceptible hybrids during the last few weeks of the growing season.  The same fungicide treatments recommended to manage gray leaf spot (above) in the late stages of corn development apply to northern corn leaf blight. 


Microscopic view of southern and common rustsNorthern corn leaf blight symptoms on a corn leaf
Figure 3. Even at 200x (left) and 400x magnification the differences between common and southern rust are subtle. (Photos by Kevin Korus)

Southern Rust

southern rust on a corn leaf
Figure 4. Southern rust pustules on a corn leaf.

We continue to identify southern rust (Figure 3) in a few new locations in Nebraska and it is reported to be increasing somewhat in other areas. The cooler temperatures, especially night temperatures, will likely help to keep its severity less than expected, making southern rust less of a threat to Nebraska corn that has reached the dent stage (R5). The confirmed distribution of southern rust can be observed on the IPM PiPE website

The most reliable method for identifying corn rust diseases (Figure 4) is based on examination of microscopic spore characteristics . This can be done quickly for samples submitted to the UNL Plant & Pest Diagnostic Clinic.  Southern rust spores are usually orange/tan in color, produced mostly on the top side of the leaf, and easily wiped off.  They can easily be mistaken for similar looking diseases, especially common rust (Figure 4). 

More Resources

A list of foliar fungicides labeled for use on corn in Nebraska and their characteristics are summarized in the 2015 Guide for Weed Management with Insecticides and Fungicides. Results from foliar fungicide trials conducted in Nebraska are available at the Crop Watch website . 

Other resources linked below are available to help identify diseases and track the Nebraska counties where southern rust has been confirmed. 

Literature Cited

Abendroth, L.J., Elmore, R.W., Boyer, M. J., and Marlay, S. K.  2011.  Corn Growth and Development.  PMR 1009. Iowa State University Extension, Ames, Iowa


Mention or display of brand names is for identification purposes only. No endorsement or criticism is intended for those mentioned or any equivalent products not mentioned.

Tamra Jackson-Ziems
Extension Plant Pathologist

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