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Different leaf diseases may have similar symptoms, particularly during the early stages of disease development. It is not uncommon for a corn plant to have several different diseases present at the same time. Multiple diseases present on a corn plant can make disease diagnosis very difficult. Laboratory culturing and microscopic examination may be required to accurately identify a leaf disease.
Figure 1. Anthracnose leaf blight.
Oval to irregular-shaped water-soaked lesions on youngest leaves turn tan to brown often with yellow to reddish brown borders. Small, black hair-like structures (called setae) may sometimes be seen in the middle of lesions. Heavily infected leaves wither and die. The organism thrives in warm, humid weather. The same fungal pathogen can also cause stalk rot; however, the presence of leaf blight does not indicate that stalk rot will occur later in the season. The stalk rot phase is of greater concern than the leaf blight phase.
Figure 2. Goss’s wilt.
Originally called leaf freckles and wilt, Goss’s wilt causes systemically infected seedlings to wilt and die. Vascular bundles can be discolored. More common later-season infections of leaves produce dull gray-green to necrotic lesions often with irregular margins. Small, water-soaked “freckles�? appear within developing lesions at the margins of lesions. Bacterial droplets may ooze from infected tissues early in the morning leaving a shellac-like shiny appearance when dried on leaf surfaces. Plant injury, such as hail or wind damage, enhances infection.
Figure 3. Gray leaf spot.
Gray to tan, rectangular lesions on leaf, sheath, or husk tissue. Spots are opaque and long (up to 2 inches). Lower leaves are affected first, usually not until after silking. Lesions may have a gray, downy appearance on the underside of leaves where the fungus sporulates. The organism thrives in extended periods of warm, overcast days and high humidity. Gray leaf spot has become more prevalent with increased use of reduced tillage and continuous corn.
Figure 4. Northern corn leaf blight.
Long (up to 6 inches), elliptical to cigar-shaped, gray-green lesions that become tan-brown are symptomatic of infection by this fungus. Infection spreads from lower leaves moving up the plant. The disease is favored by high humidity and moderate temperatures. Lesions may form in bands across leaves as a result of infection in the whorl.
Figure 5. Southern rust.
Small, circular, orange-colored pustules occur on upper surfaces, leaf sheaths, and husk leaves. Pustules often are very dense in areas of infected tissues. Pustules break the leaf surface (epidermis) less frequently than common rust. This organism is favored by warm, humid weather.
Timely scouting is important to help protect corn plants from diseases. Since much of a corn plant’s energy from photosynthesis is produced by the leaves immediately surrounding the primary ear and above the ear leaf, those leaves should be protected from foliar diseases. Fungicide applications made before a disease spreads throughout the corn canopy may help maximize yield potential under environmental conditions that result in high disease pressure. However, fungicides will have no effect in preventing bacterial diseases such as Goss’s Wilt.
Fields with foliar diseases should be scouted for stalk health as the reduction in photosynthesis can predispose corn plants to stalk lodging. Identification of foliar diseases can help determine the need for management practices such as tillage, crop rotation and the selection of more resistant corn products to help reduce disease occurrence next season.
Seed products with DEKALB® Disease Shield™, a new seed breeding effort to mitigate the effect of diseases on yield potential, will be available for the 2017 growing season. These products provide genetics with strong disease tolerance to five of the most common corn diseases: Anthracnose stalk rot, Goss’s wilt, Gray leaf spot, Northern corn leaf blight, and Southern rust. Consider discussing with your seed representative how DEKALB® Disease Shield™ seed products can reduce corn disease risk in your operation.
1 Corn Diagnostic Guide. 2014. Monsanto Company. https://www.aganytime.com/dekalb/tools/Documents/CornDiagnosticGuide.pdf/. 2 White, D.G. 1999. Compendium of corn diseases. Third edition. American Phytopathological Society Web sources verified 06/16/16. 160615184454.