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Yield loss can be incurred when leaves in the middle and upper portion of the canopy, two leaves below the ear and above the ear are infected with disease because these leaves contribute at least 75% of the carbohydrate content of the ear. Disease and other stress factors can reduce the photosynthetic capacity of corn plants during the critical grain-filling period. Decreasing the photosynthetic rate of leaves after tasseling can reduce kernel survival and kernel weight. In addition, when the photosynthetic capacity of the plant is reduced, kernel demand for products created by photosynthesis can increase remobilization of stored carbohydrates from stalk and leaf tissue. Stalks can be weakened by this process, increasing plant susceptibility to stalk and root rots. Multiple stress factors during grain fill can have a significant impact on corn yield potential.
Foliar corn diseases tend to occur during periods of warm, humid or cool, overcast weather with extended periods of dew or free-standing moisture on the leaves and crop residue in the field. Northern corn leaf blight (NCLB) infection is favored by temperatures between 66 and 80 °F, accompanied by extended periods of wetness, recent moderate temperatures and frequent rainfall. Grey leaf spot (GLS) and southern rust infections occurduring prolonged warm (75° to 85°F), humid (more than 90 percent relative humidity) periods. Symptoms of GLS are commonly observed following long periods of heavy dew and overcast days and in bottomlands or fields adjacent to woods where humidity will be higher and dew will persist longer into the morning.
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.
Thresholds for fungicide use for NCLB do not exist; however, it is especially important to protect the ear leaf and those above it as corn plants enter reproductive growth stages. Consider using a fungicide on corn products that are susceptible to NCLB or GLS if disease symptoms are present on the 3rd leaf below the ear or higher on 50% of the plants examined.
Northern Corn Leaf Blight. Typical symptoms of NCLB are large (1 to 6 inches long) cigar-shaped lesions. (Figure 1) Lesions are initially gray-green with awater-soaked appearance and turn brown as infected tissues die. A distinct margin between the infected and healthy tissue often is apparent. Distinct dark areas of fungal sporulation develop within necrotic lesions when weather is humid. Mature N
CLB symptoms can look similar to the leaf blight phase of Goss’s wilt.
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 3). 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. 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.
Southern Rust. Pustules develop primarily on the upper surface of leaves and only sparsely on the lower leaf surface (Figure 4). Pustules are circular to oval in shape and light orange in color. These pustules erupt and expose small, dust-like spores, which are dispersed by wind. Unlike common rust, pustules may also develop on ear husks, and leaf sheath tissue surrounding the stalk.
Triazole and strobiliurin fungicides are labeled for corn to help manage foliar fungal diseases. The Corn Disease Working Group has developed efficacy ratings for most of the corn fungicides at: https://extension.tennessee.edu/publications/Documents/W342.pdf
Triazole fungicides interfere with fungal membrane structure and function and must be applied preventively or as early-infection treatments. Following application they move locally into the leaf but are not necessarily transported to other leaves. Strobilurin fungicides inhibit fungal respiration and should be applied preventively or as early as possible in the disease cycle. They are absorbed into the leaf and have some upward movement in the xylem. Most triazoles and strobilurins have some residual activity based on rate of application, coverage and environmental conditions. Consult individual product labels for harvest interval and other restrictions for use.
In most cases, fungicide applications should be applied at or after tasseling. Fungicides applied from tasseling to early silking tend to have the best possibility for economic return. Do not use adjuvants if an application will be made prior to the VT (tasseling) growth stage. Follow all individual product label instructions for proper application timing, application volume, application equipment, environmental, and harvest interval precautions.