Delayed corn planting due to excessive spring rains is creating many challenges for farmers.

The extended planting window, which has delayed growth and maturity well beyond where the corn crop would be in a more “typical” year, will likely increase the incidence of yield-inhibiting foliar diseases.

When corn plants battle key foliar diseases during critical maturity stages pollination through grain fill — plant vigor and ear and kernel formation is likely compromised, which ultimately affects yield.

Foliar Fungicides

Fungicide applications can be an effective component of an integrated approach to manage foliar diseases of corn. It is important, however, to consider several factors before deciding on a fungicide application.

  • Scouting is essential. Just before tassel emergence, examine corn plants for disease symptoms from several locations in each field. Management decisions should be based on symptoms present and a field-by-field basis.
  • Know your field history of disease.
  • Know your hybrid’s susceptibility to diseases. Fungicide sprays generally are not recommended for resistant hybrids. For susceptible hybrids, fungicides should be considered if symptoms are present on the third leaf below the ear or higher on at least 50% of the plants. For hybrids with intermediate levels of disease resistance, fungicides should be considered if symptoms are present on the third leaf below the ear or higher on at least 50% of the plants, if at least 35% of the soil surface is covered with corn residue, the previous crop was corn and weather is favorable for foliar fungal diseases.
  • Proper diagnosis is critical. Fungicides do not control bacterial diseases such as Goss’s and Stewart’s wilt, so make certain to identify the disease present accurately.

Scouting Tips

Three of the most common later-season foliar corn diseases include gray leaf spot, southern rust and northern corn leaf blight. Corn growers should be well-versed in how to scout effectively and accurately identify these common foliar diseases.

Keep in mind that because of the erratic weather and delayed planting season, fields will vary dramatically in stage of maturity. Growers should be diligent about scouting often and refer to planting date records for individual fields.

The following scouting and identification tips will be helpful in making accurate diagnosis and determining the most effective treatment option, if warranted.

Southern rust: Rusts produce distinctive structures called pustules — 0.2 to 2 mm in diameter — that erupt through the surface of leaves, stalks or husks and produce spores. Three main characteristics distinguish southern rust from common rust: southern rust pustules are smaller and orange-brown while common rust pustules are larger and brick-red; southern rust pustules are more densely clustered; and southern rust pustules occur primarily on the upper leaf surface. Pustules become dark in color later in the season.

Severe infections can impact yield and cause defoliation and premature senescence. Risk of yield loss is greater in late-planted or long-season corn. Consider applying a fungicide labeled for southern rust is disease is at a high enough level early in the growing season.

Farmers who live in the southern portions of Illinois and Indiana can follow Nathan Kleczewski at @ILplantdoc and Corn Diseases at @corndisease on Twitter for updated information on disease tracking.

Northern corn leaf blight: This common foliar disease is typified by long 1- to 6-inch lesions with tapered ends that are gray-green to tan in color. Disease usually begins on lower leaves, but can spread to all leaves and husks with secondary infections. When lesions coalesce, the entire leaf can become flighted and symptoms resemble frost-killed leaf tissues. Infection occurs when conidia are exposed to six to 18 hours of leaf wetness and moderate 65- to 80-degree temperatures. Susceptible hybrids and high-nitrogen soils increase disease risk.

Several fungicides are labeled for northern corn leaf blight control. Foliar fungicides may be effective when applied early — when the first lesions appear on lower leaves — and when weather conditions are favorable for disease.

Gray leaf spot: Mature lesions are rectangular, approximately 3 to 4 inches, are bordered by leaf veins and appear tan to brown in color turning silvery-gray when sporulating. On some hybrids, lesions may be orange to yellow and may be evident on the leaf sheath and stalk. Early lesions are small, necrotic spots and may have a chlorotic halo, which is more visible when leaf is backlit.

The fungus overwinters on corn debris, and practices such as conservation tillage and continuous corn increase risk. Spores begin to develop on corn residue in response to warm, humid weather. Fields located in river bottoms or in weedy fields have an increased risk for disease.

Scout a couple of weeks before tassel emergence. Fungicides applied before significant damage has occurred may be economical in fields with history of significant yield loss due to gray leaf sport or in high-value corn. If high levels of disease develop during grain fill, early harvest may be warranted.

Next Season

Several management practices have proven effective in controlling or minimizing the incidence of foliar diseases in corn for the next growing season:

  • If spring weather conditions allow, earlier planting dates and/or shorter-season hybrids may reduce risk of yield loss.
  • Plant resistant hybrids if available.
  • Put a foliar fungicide into your crop management plan.
  • Rotate to another crop.
  • The northern corn leaf blight pathogen overwinters in corn residue. Residue that resides on or near the soil surface can increase the risk of the disease. In fields with a history, employing the use of continuous and no-till corn production will increase the risk of the disease. Residue management through tillage and crop rotation can help to destroy primary inoculum.

For more information, contact your local county Extension educator, or the University of Illinois Department of Crop Sciences at

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