Basic Corn Diseases

Each growing season brings its own disease challenges. This year, many areas across the Midwest have experienced periods of frequent rainfall, high humidity, and extended leaf wetness—conditions that can favor the development of several common corn diseases. While disease pressure varies by geography, hybrid, and field history, regular scouting remains the best way to identify issues early and make informed management decisions.

Corn diseases can vary greatly depending on the pathogen, environment, farming and tillage practices, hybrid selection, and weather patterns throughout the growing season. It is important to proactively scout corn fields for disease, as crop yield and quality can be significantly impacted in areas with high disease pressure.

For a disease to develop, all three components of the disease triangle must be present: a susceptible host, a pathogen, and a favorable environment.

When all three conditions of the disease triangle are present, disease development is likely to occur. However, disease presence does not always mean treatment is necessary. Management decisions should be based on disease severity, field conditions, yield potential, and the likelihood of economic return.

As corn moves into the reproductive stages, protecting leaf area becomes increasingly important. Diseases that develop on or above the ear leaf can reduce photosynthetic capacity during grain fill and ultimately impact yield. Regular scouting can help determine if disease severity and environmental conditions justify additional management.

When scouting your fields, use the following information as a general guide to identify symptoms of common corn diseases:

Common Rust

• Fungal pathogen; occurs very frequently.
• Prefers cool temperatures (60–76°F) and at least 6 hours of leaf moisture for infection to occur.
• Spores are wind blown from Mexico and the Gulf Coast states.

• Produces random, dark reddish-brown pustules on both the upper and lower leaf surfaces.

• Management: Resistant hybrids and fungicide application when warranted. 

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Southern Rust

• Fungal pathogen; somewhat common late in the growing season.

• Prefers warm temperatures (77–82°F) and at least 6 hours of leaf moisture.

• Spores are windborne from Mexico and the Gulf Coast states.

• Produces small, orange to brown pustules clustered on the upper surface of the leaf, often surrounded by a yellow halo.

• Management: Resistant hybrids and fungicide application when warranted. 

Common Smut

• Fungal pathogen; very common.

• Overwinters in the soil, and spores can be spread by wind.

• Spores can germinate throughout the growing season at temperatures ranging from 50–95°F.

• Rarely causes severe damage but can become severe, especially on young plants following an injury.

• Forms smut galls on any aboveground part of the plant.

• Management: Resistant hybrids, crop rotation, proper soil fertility, and tillage. 

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Eye Spot

• Fungal pathogen; fairly common.

• Overwinters on corn residue, with spores dispersed by wind.

• Infection occurs during cool, wet weather.

• Produces small, circular lesions (approximately 1/8 inch in diameter) with tan centers, surrounded by a brown to purple ring and a yellow halo that is most visible when the leaf is held up to the light.

• Disease is most severe in no-till or reduced-tillage systems where crop residue remains on the soil surface.

• Management: Hybrid resistance, residue management, crop rotation, and fungicide application when warranted. 

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Gray Leaf Spot

• Fungal pathogen; common.
• Overwinters on corn residue at soil surface.
• Infection occurs during prolonged warm and humid conditions.
• 75-90°F, 90%+ humidity, and 12 hours leaf wetness.
• Can become economically damaging in corn on corn, no-till fields.
• Early infection creates small, pin point lesions that can be confused with eye spot and common rust.
• As pathogen develops, lesions become enlarged, turn brown-gray, and are bound between leaf veins.
• Management: Resistant hybrids, residue management, crop rotation, fungicide if needed.

Northern Corn Leaf Blight

• Fungal pathogen; common.

• Overwinters on corn residue at the soil surface.

• Spores spread quickly by wind and rain splash.

• Infection occurs when leaves remain wet for 6–18 hours and temperatures range from 65–80°F.

• Lesions are cigar-shaped, measuring 1–6 inches long, and are gray-green to tan in color.

• Symptoms typically begin on the lower leaves and progress upward to the upper leaves and husks.

• Yield losses can range from 30–50% when infection occurs prior to tasseling.

• Management: Resistant hybrids, fungicide application, and crop rotation. 

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Goss’s Wilt

• Bacterial pathogen; very common.

• Overwinters in crop residue on the soil surface.

• Has many hosts, including green foxtail and barnyard grass.

• Bacteria spread by splashing onto the lower leaves of corn plants, typically entering through wounds caused by wind, hail, insect feeding, or other injuries.

• Disease development is favored by warm, humid conditions with temperatures of 80°F or higher.

• Symptoms appear as either foliar blight or systemic wilt.

• Lesions are typically gray to tan with wavy, irregular margins that follow the leaf veins.

• Dark green to black, water-soaked spots—often called "freckles"—may be present within the lesions.

• Bacterial ooze may be visible on the leaf surface.

• Management: Hybrid tolerance, crop rotation, tillage, and control of grassy weeds.

• No in-season management options are available. 

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Holcus Leaf Spot

• Bacterial pathogen; somewhat common.

• Overwinters in crop residue.

• Wounds caused by wind or hail can increase disease severity.

• Warm (75–85°F), wet, and windy conditions early in the growing season favor disease development.

• Has a wide range of hosts, including many grasses and dicot species.

• Symptoms first appear as water-soaked, dark green lesions.

• Lesions develop into round, tan to white spots measuring 1/8–1/2 inch in diameter.

• The spots are surrounded by red to brown margins and a yellow halo.

• Management: Crop rotation and tillage.

• No in-season management options are available. 

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Conclusion

No two growing seasons are alike. Staying ahead of disease starts with routine scouting, understanding hybrid characteristics, and responding to current field conditions. When questions arise, your local Legend Seeds agronomist can help identify disease concerns and determine the best management strategy for your operation.

Content and Photo Credit: SDSU iGrown, Corn Diseases in South Dakota and Their Management and http://cropdisease.cropsci.illinois.edu/corn