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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, which can make diagnosis very challenging. Laboratory culturing and microscopic examination may be required to accurately identify a leaf disease.
Symptoms include gray to tan, rectangular lesions on leaf, sheath, or husk tissue. Spots are opaque and long (up to 2 inches, Figure 1). 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 1. Gray leaf spot.13;10;
Small, circular, orange-colored pustules occur on upper surfaces, leaf sheaths, and husk leaves (Figure 2). 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.13;10;
Figure 2. Southern rust.13;10;
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 (Figure 3). 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. Goss’s wilt leaf freckles (top). Leaf freckles appear luminous when held up to block the sun (bottom).13;10;13;10;
Symptoms occur at any stage of corn growth, but typically have been observed first on lower leaves spreading to the middle and upper portion of the crop canopy after flowering. Initial symptoms first appear as translucent, water-soaked streaks between veins (Figure 4) and progress to longer yellowish to necrotic steaks that can coalesce to form larger areas of symptomatic tissue. Bacterial exudates on the leaf surface may appear as small, dried, yellow-colored droplets.
Figure 4. Bacterial leaf streak.13;10;13;10;
Small yellow spots appear first at the base of the leaf and over time turn brown in color. As infection progresses, spots can often be found occurring in bands across the leaf. Spots in the mid-rib of the leaf become reddish to brown in color and combine to form irregular blotches (Figure 5). Sheath, husk, tassel, stalk, and leaves may exhibit symptoms late in the season. Infected stalks may break at a node. This fungus is favored by warm, wet weather.
Figure 5. Physoderma brown spot.
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 and bacterial leaf streak.
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.
Consider selecting seed products with DEKALB® Disease Shield™, a seed breeding effort to mitigate the effect of diseases on yield potential. 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.