Update on corn diseases and fungicide decisions in Iowa

July 21, 2021 4:37 PM
Blog Post

If you listen carefully, you can hear corn tassels rattling in the breeze, and occasionally the hum of a crop sprayer. 

This growing season started off exceptionally dry, apart from southeast Iowa. The moisture that arrived in July and hung around for a couple of weeks was welcomed by most growers, unless it was accompanied by hail or tornados! It was also welcomed by fungal pathogens that like to hang out in corn fields. 

Not surprising, some foliar diseases have started to show up in corn.  I have seen or heard reports of gray leaf spot, northern corn leaf blight and tar spot (Figure 1a, b and d, respectively).  To the south and west of Iowa, southern rust is being reported (Figure 1c). All of these diseases can be managed in-season with the applications of a fungicide.  But when should you spray?  Are there disease thresholds? Is there a particular growth stage?

Corn leaves showing early symptoms of gray leaf spot, northern leaf blight, rust and tar spot

Figure 1.  Early symptoms of gray leaf spot (a), northern corn leaf blight (b), tar spot (c) and common (top) and southern (bottom) rust (d).

The problem with disease for farmers is yield loss. The problem with disease for pathologists is predicting how it is going to develop.  Much as pathologists love the disease triangle, it can also be frustrating.  Seeing the first symptoms of disease in a field indicates three sides of the triangle were present a few to several days previously, but if one of those sides (in this case, environment) then becomes absent, disease will not develop further, until all three sides are present again. 

Table 1 shows the environmental conditions necessary for several foliar diseases to develop in corn. When conditions yo-yo back and forth between wet and dry it is difficult to predict how disease will develop and come up with disease thresholds or to decide if a fungicide application is necessary to prevent further disease developing.

Table 1. Favorable conditions for leaf spot development on corn

  Temperature Moisture

Common rust

61-77 F

>6 h of leaf wetness

Gray leaf spot

77-88 F

11-13 h dew or fog;

relative humidity in the canopy >90% for 12h

Northern corn leaf blight

65-75 F

>6 h of leaf wetness

Southern rust

77-82 F

>6 h of leaf wetness

Tar spot

60-70 F

>7 h of leaf wetness

In general, we see the best return on investment on a fungicide application when disease is present in a field on the ear leaf and in the upper canopy of most plants throughout the field at R5.  Since most fungicides are applied at or within two weeks of tasseling, this further complicates the decision to spray a fungicide. Taking into consideration the susceptibility of the hybrid to disease, weather forecast and microclimate of the field, cropping and disease history of the field, and the balance in your checking account can help with a management decision. If it looks like all three sides of the triangle are, or will be, present through grain fill, it may be prudent to spray.  For the next 30 days, the Climate Prediction Center suggests it will be warmer than normal with below normal precipitation.

Corn pathologists across the U.S are working together to develop models that can be used to predict disease and help determine when a fungicide should be sprayed. You may visit AgPMT.org to learn more. In the next few years, we plan to develop models to predict disease development through this project. The Tarspotter app, developed by Damon Smith at the University of Wisconsin is a good example; it uses a model to predict the risk of tar spot in a field, and the need for a fungicide application.

Additional resources:

Impact of foliar fungicide timing and fungicide class on corn yield response in the United States and Canada

Fungicide efficacy for control of corn diseases

ISU Research Farm Reports: Effect of foliar fungicides on hybrid corn


Acknowledgements: Thanks to Rebecca Vittetoe for reviewing earlier drafts of this blog.




Alison Robertson Professor of Plant Pathology and Microbiology

Dr. Alison Robertson is a professor of plant pathology and microbiology. She provides extension education on the diagnosis and management of corn and soybean diseases. Her research interests include Pythium seedling disease of corn and soybean and Goss's wilt. Dr. Robertson received her bach...