Corn and Soybean Diseases

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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 identify a leaf disease.

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Typical symptoms of Northern corn leaf blight (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 can look similar to the leaf blight phase of Goss’s wilt.

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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.

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Gray Leaf Spot. Gray to tan, rectangular lesions on leaf, sheath, or husk tissue (Figure 3). It can be confused with other foliar diseases of corn such as newly discovered bacterial leaf streak (Figure 4). 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.

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Foliar symptoms of Brown stem rot (BSR), and Sudden death syndrome (SDS) can look similar and include chlorosis and browning of the tissue between the veins (Figure 5). Wet weather and moist soil conditions favor disease infection and development. These foliar disease symptoms may also look like early crop maturity and drought stress. Distinguish between SDS and BSR by splitting the stem and looking at pith coloration. The pith will be discolored with BSR while the pith will be white with SDS (Figure 6). SDS usually shows up in scattered areas throughout soybean fields. The disease symptoms generally appear as soybeans develop pods.

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White mold produces white, cottony mycelial growth on the outside of the stem and pods, wilted leaves, and stems that appear “bleached�? and shredded. Sclerotia are small black structures found on and inside plants that have been infected by white mold. White mold is the only disease discussed here that has in-season management options.

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