Figure 7. Lesions of southern corn leaf blight – race 0 are much smaller than those of northern corn leaf blight. Lesions are usually 1 to 2 inches long with limited parallel borders that are buff to brown in color.
The first step in management of corn leaf diseases is use of disease-resistant hybrids. Although no single hybrid is resistant to all diseases, hybrids with resistance to most major leaf blight, stalk rot, and viral diseases are available. Disease resistance does not necessarily mean immunity to disease. Resistance is commonly seen as a reduced number or size of leaf spots and/or a reduction of spread of the pathogen in the field. Some hybrids are resistant to one or several races of a pathogen, but are very susceptible to other races of the same pathogen. The pathogens causing northern corn leaf blight, southern corn leaf blight (Figure 7), and northern leaf spot (Figure 8) exist as several different races. Therefore, it is sometimes important to know the race(s) of the pathogen prevalent in your area. When a new race develops and becomes widespread, previously resistant hybrids appear susceptible.
It is especially important to suppress leaf diseases until 4 to 5 weeks after tasseling so that leaves above the developing ear do not become blighted, because the ear leaf and upper leaves contribute to filling the grain. A great deal of blighting may occur on leaves below the ear without reducing yield. Additionally, when severe leaf blight occurs after the beginning of kernel dent stage, yield losses are generally low. Figure 9 presents a commonly used scale for assessing the level of disease on corn plants. Figure 10 illustrates a method of assessing differing levels of disease severity on individual leaves where data are expressed as a percentage of leaf area affected. The percentage leaf area affected on the ear leaf is commonly used to assess gray leaf spot severity using the scale in Figure 11. High-yielding hybrids should be selected that have resistance to the diseases most common and severe in your area. Hybrids with resistance to northern corn leaf blight, southern corn leaf blight, Stewart’s bacterial leaf blight, anthracnose, and common rust (Figure 12) are needed across the state. Under reduced tillage, especially in continuous corn, resistance to gray leaf spot, anthracnose, northern leaf blight, eyespot (Figure 13), and northern leaf spot are needed. Hybrids with resistance to common smut (Figure 14) and several other less important diseases are also available.
 Figure 8. Northern leaf spot, caused by Bipolaris zeicola race 3, can be recognized as narrow linear lesions that run parallel with veins of the leaf. These small lesions are usually grayish-tan surrounded by a dark pigmented border. |
 Figure 12. Common rust is more important during cool summers with high relative humidity. Small, oval pustules filled with orangish-red spores are a distinctive sign of common rust. |
 Figure 10. A diagramatic representation of the percentage of leaf area covered by lesions of southern corn leaf blight. Illustrations such as this are used to evaluate disease severity on corn leaves. From: James, C. 1971. A manual of assessment keys for plant diseases, The American Phytopathological Society, 3340 Pilot Knob Rd., St. Paul, MN 55121. |
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 Figure 11. Scale used to assess the percentage of leaf area affected by gray leaf spot. From: Smith, K.I. 1989 Epidemiology of gray leaf spot of field corn caused by Cercospera zeae-maydis, Ph.D. thesis, University of Maryland. |
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 Figure 13. Eyespot lesions are very small and round with white centers and dark margins. Eyespot is important only in continuous corn under reduced tillage management. |
 Figure 14. Common smut or boil smut is a highly visible disease found in corn fields and in home-owner sweet corn gardens. The unsightly smut boils enlarge within days after infection of ears, tassels, stalks, or leaves. As the boils mature, the contents change into masses of black spores. Since no toxin is associated with this fungal disease, infected plants can be fed to livestock. |
Copyright © 2004, Ohio State University Extension
All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.
Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension
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