How to Identify Common Corn Diseases in the Field

By Blake Jackson

Proper disease identification is essential for corn growers when deciding whether fungicide applications are needed or evaluating the success of previous treatments, according to Penn State Extension specialists. Adriana Murillo-Williams, Alyssa Collins, and Paul D. Esker emphasize that correct scouting practices are a key part of an effective Integrated Pest Management (IPM) program.

Foliar diseases can negatively impact corn production by reducing photosynthetic capacity, lowering grain yield, affecting silage quality, increasing stalk rot risks, and causing premature plant death.

Regular field scouting helps growers determine whether diseases or disease-carrying pests are present, whether crops are at vulnerable growth stages, and if disease severity has reached levels that require action.

Disease pressure often increases during grain filling when warm, humid conditions continue. Common diseases such as gray leaf spot and northern corn leaf blight can spread quickly in susceptible hybrids, especially in no-till and continuous corn systems.

Growers should also monitor emerging threats like southern rust and bacterial leaf streak, which can resemble other diseases but require different management strategies.

Important corn diseases and identification features include:

• Tar spot: Appears as raised, shiny black spots on leaf surfaces that cannot be removed by rubbing and may include surrounding tan or brown “fish-eye” lesions.
• Northern corn leaf blight: Produces long, cigar-shaped gray-green to tan lesions that develop along leaf veins.
• Gray leaf spot: Creates rectangular gray or tan lesions that expand parallel to leaf veins.
• Northern corn leaf spot: Often causes small tan-to-brown lesions arranged in a “string of pearls” pattern.
• Common rust: Forms brown to brick-red pustules on both leaf surfaces and is usually less damaging due to hybrid resistance.
• Southern rust: Produces bright orange pustules mainly on upper leaf surfaces and can cause significant yield losses.
• Bacterial leaf streak: Creates narrow brown streaks between leaf veins and cannot be controlled with fungicides.
• Corn stunt: A disease complex spread by corn leafhoppers that can cause yellow striping, plant stunting, and poorly developed ears.

Accurate diagnosis helps growers choose appropriate management practices, avoid unnecessary treatments, and protect crop productivity throughout the growing season.

FAQs

• Why is corn disease identification important?

It helps growers select effective treatments and prevent unnecessary fungicide applications.

• When should corn fields be scouted for diseases?

Regular scouting is especially important during grain filling and humid weather conditions.

• Which corn diseases are commonly found in fields?

Gray leaf spot, northern corn leaf blight, rusts, tar spot, and bacterial leaf streak are major concerns.

• Can fungicides control bacterial leaf streak?

No, fungicides are ineffective because bacterial leaf streak is caused by bacteria.

• What conditions increase corn disease risk?

High humidity, moderate temperatures, susceptible hybrids, and continuous corn production increase risk.

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