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Disease development in corn around the tasseling stage of growth can result in yield loss. Disease development is favored by warm, humid or cool, overcast weather with extended periods of dew or free-standing moisture that supports continued infection of leaves around and above the ear. Yield loss can be incurred when the 2 to 3 leaves above and below the ear are infected with disease because these leaves contribute at least 67% of the carbohydrate content of the ear. Disease and other stress factors can reduce the photosynthetic capacity of corn plants during the critical period of grain filling.1 Several leaf diseases may have similar symptoms, particularly during the early stages of disease development. Laboratory culturing and microscopic examination may be required to accurately identify a leaf disease.
Figure 1. Elliptical or cigar-shaped lesions typical of NCLB.
Northern Corn Leaf Blight. Typical symptoms of NCLB are large (1 to 6 inch long) cigar-shaped lesions. Lesions are initially gray-green with a water-soaked appearance and turn tan-brown as infected tissues die (Figure 1). A distinct margin between the infected and healthy tissue often is apparent (Figure 1). Distinct dark areas of fungal sporulation develop within necrotic lesions when weather is humid . Mature NCLB symptoms can look similar to the leaf blight phase of Goss’s wilt.
Figure 2. Goss’s wilt leaf freckles appear luminous when held up to block the sun.
Goss’s Wilt. The leaf blight symptoms of Goss’s wilt usually appear as long, gray-green to black, water-soaked streaks extending along leaf veins (Figure 2). Small, dark, water-soaked flecks, referred to as “freckles�?, often occur inside larger lesions and at the edges of lesions where symptoms are advancing. Leaf freckles are luminous when lighted from behind, such as when the sun is used as backlighting. Bacterial cells may ooze from infected leaves and dry on leaf surfaces forming a shellac-like sheen. As lesions mature, large areas of tan to brown dead leaf tissues are visible. Fungicides are ineffective against Goss's wilt which is caused by a bacterium.
Figure 3. Gray leaf spot lesions.
Gray Leaf Spot. Gray to tan, rectangular lesions on leaf, sheath, or husk tissue (Figure 3). 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. GLS has become more prevalent with increased use of reduced tillage and continuous corn.
Begin scouting fields for foliar disease symptoms just before tasseling and continue through the grain filling stages of growth. Examine the ear leaf and leaves above and below the ear at several locations through a field. If disease is present on a majority of the leaves, a fungicide application may be necessary. Consult with your agronomist to determine appropriate actions to address the disease situation.
1Nielsen, R.L. 2013. Stress during grain fill: A harbinger of stalk health problems. Purdue University. 160701084406