Common Corn Diseases You Need to Know

When scouting corn fields, proper disease identification is critical for determining whether a fungicide application is warranted.

Identifying Corn Diseases

Ofentimes “lesions” or “spots” can look like many other pests or diseases. Additionally, some non-yield limiting pathogens can produce symptoms similar to yield-limiting pathogens. This article discusses some of the major yield-limiting diseases and how to properly identify each. 

What is Common Rust?

Common Rust is an airborne fungus that overwinters in southern states and spores are wind-blown to the northern states. This disease usually starts to appear in late June and is favored by relative humidity greater than 95%. It is also common with heavy dews resulting in 6 or more hours of leaf wetness and in cooler temperatures of 65-76 degrees Fahrenheit. 

What does Common Rust look like?

The disease is best identified by observing brownish-red oblong pustules that occur on upper and lower leaf surfaces, husks, leaf sheaths and or stalks. New infection can occur every 7-14 days under favorable conditions. 

What is Southern Rust? 

Similar to Common Rust, Southern Rust is an airborne fungus that overwinters in southern states and relies on wind currents to be moved north. Of the two rust diseases, Southern Rust is more threatening to producers when it comes to negatively impacting corn yields. 

Unlike Common Rust, Southern Rust prefers higher temperatures of 80-90 degrees Fahrenheit. Therefore, it generally appears in later June to July/August. 

What does Southern Rust look like? 

The pustules of Southern Rust are generally small, circular or oval in shape with a light green to yellow halo. Mature pustules erupt and release spores that are orange in color (Tennessee Volunteer Orange).  

Southern Rust primarily develops on the upper leaf surface; however, it can develop on stalks, husks, and leaf sheaths under heavy pressure. New infections can occur every 7 days under favorable conditions.

What is Gray Leaf Spot (GLS)? 

Gray Leaf Spot first appears on the lower leaves of young corn due to spores being splashed from residue. The disease can also spread by wind, carrying spores from one cornfield to another. It overwinters in corn residue, so the rise in no-till acreage has contributed to its persistence.  

What does Gray Leaf Spot (GLS) look like? 

GLS lesions are rectangular in shape as they are restricted by corn leaf veins (see below). Under favorable conditions, lesions can coalesce to form large, irregular lesions (see below). GLS development occurs during prolonged warm (75-85 degrees Fahrenheit) humid (>90% relative humidity) days. 

What is Northern Corn Leaf Blight (NCLB)  

This disease's development is worsened by over 6 hours of leaf wetness and moderate temperatures ranging from 65-82 degrees Fahrenheit. Under these conditions, lesions can develop 7-12 days after infection.  

NCLB can easily be confused with leaf injury as a result of foliar nutrient application and in some rare cases be confused with leaf scald or drought. 

What does Northern Corn Leaf Blight (NCLB) look like? 

Northen Corn Leaf Blight are large, rounded cigar-shaped lesions. Lesions range from 1 to 7 inches in length. Unlike GLS, they are not restricted by corn leaf veins (see image below). This fungus overwinters in corn residue and is dispersed by wind and splashing water.

What is Goss’s Bacterial Wilt? 

Goss’s Wilt is caused by a bacterial pathogen which means it cannot be controlled with a fungicide application. This disease can be managed by using resistant hybrids, crop rotation to a non-host crop and tillage. Goss’s Wilt overwinters in corn residue and initial infection occurs from rain splashing spores onto corn plants.  

Disease development is favored by warm temperatures (>80 degrees Fahrenheit) and it commonly enters corn plants through wounds created by wind, blowing dust particles, sand or hail. Insect feeding is another factor linked to the development and spreading of Goss’s.  

What does Goss’s Bacterial Wilt look like? 

Symptoms of Goss’s Bacterial Wilt are elongated lesions with irregular margins. This includes dark, water-soaked freckles that develop in the lesion as well as shiny patches of dried bacterial ooze that appear similar to dry varnish. 

What is Tar Spot? 

Since 2015 when Tar Spot was first reported in the Midwest United States, it has quickly become the #1 yield-robbing disease on corn in the United States and Canada. The fungus overwinters in corn residue and spreads by wind and splashing rain. This disease thrives in cool (60-70 degrees Fahrenheit), humid (>75% relative humidity), and prolonged leaf wetness (>7 hours).  

What does Tar Spot look like? 

The fungus produces small (1/16-3/4 inch), round to irregular diamond-shaped stromata, which appear as raised black bumpy structures (like a small drop of tar on the leaf) that do not rub off. These stromata can form on both the upper and lower surfaces of corn leaves. In severe cases, stromata even develop on husks, stalks, and leaf sheaths.

Yield Loss

The table below shows estimated annual corn yield losses (millions of bushels) due to diseases in 29 U.S. corn producing states.

Yield losses due to tar spot can vary depending on the time of onset, weather conditions and hybrid susceptibility. Losses can be minimal to none, and up to 50 bushels per acre or more in severe cases. Timely fungicide applications are critical to reduce the severity of this disease.

The Takeaway

Scouting is important during these sporadic weather patterns that are providing a conducive disease environment. For further information or help making in-field diagnosis, Please consult any of your local Agronomic Sales Enablement Team or your local Simplot Grower Solutions Crop Advisor to answer your application questions. 

Authors

Darrin Roberts, Northern Plains
Brice Bunck, Midwest
Chris Carpenter, Great Plains
Kevin Caffrey, Eastern Cornbelt

Click to read more about the authors