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Disease development in corn around the tasseling stage of growth can result in yield loss, particularly if favorable environmental conditions support continued infection of leaves around and above the ear. Foliar diseases of corn are a concern when they develop early and progress up the plant before grain fill is complete. Warm, humid conditions with free standing water on leaves have promoted early infection of corn by Northern corn leaf blight (NCLB).1 Some other diseases to watch for include Gray leaf spot (GLS), rust, eyespot, Anthracnose leaf blight, and bacterial diseases such as Goss’s wilt. There are a number of fungicides labeled for use in corn that can help manage several of the foliar fungal diseases. Fungicides are not effective on bacterial diseases.
Yield loss can be incurred when the top 8 to 9 leaves above the ear are infected with disease because these leaves contribute at least 75% of the carbohydrate content of the ear.2 Disease and other stress factors can reduce the photosynthetic capacity of corn plants during the critical period of grain filling.3 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. Reports of NCLB lesions occurring early on pre- tasseling corn in many parts of the Cornbelt indicates the potential for significant yield loss if favorable conditions persist.
The common denominator for foliar corn disease infection is warm, humid or cool, overcast weather with extended periods of dew or free-standing moisture on the leaves and crop residue in the field. NCLB infection occurs at temperatures between 66 and 80° F, accompanied by extended periods of wetness, recent moderate temperatures and frequent rainfall favors infection and disease development.4 GLS infection occurs during prolonged warm (75° to 85°F), humid (more than 90 percent relative humidity) periods.5 Symptoms 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. Rapid grain filling occurs from R2 (blister) to late R5 (full dent). 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 stages of growth. 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.6
Figure 1. NCLB lesions on leaves around the ear at silking.
Northern Corn Leaf Blight. 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 (Figure 1) 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. Gray Leaf Spot lesions.
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. GLS has become more prevalent with increased use of reduced tillage and continuous corn.
Figure 3. Goss’s wilt leaf blight symptoms with freckles.
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. Small, dark, water-soaked flecks, referred to as “freckles65533;?, 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 4. Anthracnose on a corn leaf.
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 is responsible for both anthracnose leaf blight and stalk rot; however, the presence of leaf blight does not indicate that stalk rot will be a problem later in the season. The stalk rot phase is of greater concern than the leaf blight phase.
Figure 5. Eyespot on a corn leaf.
Eyespot. Small (less than 1/4-inch), circular, translucent lesions surrounded by yellow to purple margins that gives them a halo effect. Lesions occur on leaves (most commonly as plants approach maturity), sheaths, and husks. The disease is favored by cool, moist weather.
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 (https://www.extension.purdue.edu/extmedia/BP/BP-160-W.pdf).7
Triazole fungicides interfere with fungal membrane structure and function and must be applied preventively or as early–infection treatments.8 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) stage of growth.9 Follow all individual product label instructions for proper application timing, application volume, application equipment, environmental, and harvest interval precautions.
There are many factors to consider when determining if a fungicide application is warranted. Prior to making an application, evaluate each field for the susceptibility of the corn products to the diseases, the current yield potential of each field, disease severity, and corn stage of development. Then consider the cost of treatment and corn price to determine if the application has a probability of providing an economical return in each field. Finally, check the weather forecast to evaluate if upcoming conditions will continue to promote disease development.
1 Robertson, A. 2014. Northern corn leaf blight prevalent in Iowa. Iowa State University Integrated Crop Management. 2 Rees, J.M. 2008. Gray leaf spot of corn. G1902. University of Nebraska. 3 Nielsen, R.L. 2013. Stress during grain fill: A harbinger of stalk health problems. Purdue University. 4 Wise, K. 2011. Northern corn leaf blight. Purdue University. 5 Wise, K. Gray leaf spot. Purdue University. 6 Bradley, C. A., Esker, P.D., Paul, P.A., and Robertson, A.E. 2010. Foliar fungicides for corn: targeting disease. University of Illinois, University of Wisconsin, Ohio State University and Iowa State University. 7 Wise, K. 2015. Fungicide efficacy for control of corn diseases. Purdue University. 8 Mueller, D. 2006. Fungicides: 8 part series. Iowa State University Integrated Crop Management. 9 Stetzel, N., Wise, K., Nielsen, B., and Gerber, C. 2011. Arrested ear development in hybrid corn. Purdue University. 150709132621