Identifying Mid-Season Corn and Soybean Diseases

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Timely scouting and disease identification in corn and soybean can provide information to facilitate crop management. Multiple diseases may be present on a corn or soybean plant at the same time, making disease diagnosis very difficult. Therefore, laboratory culturing and microscopic examination may be required to accurately identify a leaf disease.

Corn Diseases

Gray leaf spot (GLS)

Symptoms include gray to tan, rectangular lesions on leaf, sheath, or husk tissue. Lesions are blocky and stay within leaf veins, and are also 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. The disease thrives in extended periods of warm and overcast days with high humidity. Gray leaf spot has become more prevalent with increased use of reduced tillage and continuous corn.


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Figure 1. Gray leaf spot.


Northern corn leaf blight (NCLB) Long (up to 6 inches), elliptical to cigar-shaped, gray-green lesions that become tan-brown are symptomatic of infection by this fungus. Infection spreads from lower leaves moving up the plant. The disease is favored by high humidity and moderate temperatures. Lesions may form in bands across leaves as a result of infection in the whorl.


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Figure 2. Northern corn leaf blight.


Common rust

Upon infection, light green to yellow chlorotic spots are observed. As time passes, raised cinnamon-brown pustules form on the upper and lower leaf surfaces that can turn blackish in color late in the growing season. The pustules may be in bands across the leaves indicating that infection occurred while the leaf was wrapped within the whorl and are less than 1/4-inch long. When the pustules rupture, small cinnamon-brown powdery spores are released.


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Figure 3. Common rust.


Southern rust Small, circular, orange-colored pustules occur on upper leaf surfaces, leaf sheaths, and husk leaves. Pustules often are very dense in areas of infected tissues. Pustules break the leaf surface (epidermis) less frequently than common rust. This organism is favored by warm, humid weather.


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Figure 4. Southern rust.


Soybean Diseases

White mold White mold produces white, cottony mycelial growth on the outside of the stem and pods, wilted leaves, stems that appear “bleached” and shredded. Sclerotia, small black structures that resemble mouse or rat droppings, can be found on and inside plants that have been infected by white mold. Development is favored by cool, cloudy, wet, humid weather at flowering. The disease is more problematic in high-yield environments where high plant populations, narrow row spacing, and an early-closing canopy are commonly used. Some foliar fungicides are available for white mold management, but timing is critical.


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Figure 5. Soybean stem affected by white mold.


Phytophthora Plants infected with phytophthora have mid- to late-season symptoms that include: brown lesions on the roots, root rot and a brown discoloration of the stem that can extend from below the soil line up into the plant. Leaves turn yellow, wilt, and stay attached to the plant. Plants may die throughout the season. The pathogen can infest plants early in the season but not show symptoms until mid-season or later if seedlings survive the damping-off stage of the disease. Wet weather, poorly drained soils, and compaction favor the disease.


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Figure 6. Leaf and stem symptoms of phytophthora


Sudden death syndrome (SDS) Foliar symptoms typically occur after the onset of flowering. Initial symptoms of SDS are chlorotic mottling between veins. As the disease progresses, chlorotic leaf tissue turns brown as it dies and entire leaflets may shrivel in severe cases. Plants will defoliate leaving the petioles attached. Splitting the stem and taproot of a SDS-infected plant will reveal tan to light brown streaks in the cortical tissue while the pith tissue remains white or slightly cream-colored. Root rot or decreased root vigor can be seen in plants with severe foliar symptoms of SDS. If such plants are removed from moist soil, light-blue spore masses of F. virguliforme may be seen. The presence of these spore masses, along with the symptoms mentioned above, is considered diagnostic for SDS.


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Figure 7. Interveinal chlorosis and white stem piths of SDS.


Frogeye leaf spot Frogeye leaf spot is most common during the soybean reproductive growth stages (blooming through maturity) but may develop earlier in continuous soybean fields and/or under optimal environmental conditions. Symptoms initially appear as small, yellow or gray spots on the leaves. As the lesions mature, they expand (up to 5 mm in diameter) and the centers of the lesions become gray to tan with reddish-brown to purple margins. The lesions may appear to have small, dark hairs on the underside of the leaf which contain the conidia (infecting spores of the fungus). When plants are heavily infected, the lesions may coalesce.


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Figure 8. Frogeye leaf spot.


Charcoal rot Charcoal rot is caused by a pathogen that can infect soybeans, corn, and sunflowers. Premature plant death with leaves still attached is the most common symptom. A gray discoloration of root and stem tissue develops below the outer tissues. Hot, dry weather, during the reproductive growth stages in the driest areas of the field. Black speckling can be found on the lower stem.


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Figure 9. Charcoal rot.


Stem canker Symptoms of stem canker first appear during the early reproductive stages as small, red-brown lesions. Initial lesions are usually found near a lower leaf node and expand lengthwise as the season progresses. Lesions eventually girdle the stem, causing wilting and plant death.


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Figure 10. Stem infected with stem canker.