Corn Foliage Diseases
By Laura Sweets
After a slow start many corn fields have really "taken-off" and plants have increased substantially in size. Foliage diseases are showing up in scattered, isolated fields. So far low levels of Stewart's bacterial wilt, common rust and gray leaf spot have been reported. In most cases these diseases are showing up on the lower leaves to mid-canopy but are not yet on the ear leaves.
Generally speaking with the corn foliage diseases, the later in the season (especially the longer after pollination) that the foliage disease becomes established, the lower direct yield losses will be. Highest yield losses occur if diseases such as rust or gray leaf spot develop prior to pollination. Also, most of the corn foliage diseases are favored by extended periods of free moisture on the leaf surfaces. This moisture can be from rain, overhead irrigation or heavy dews that stay late in the day. Fields with poor air movement, river bottom fields or shaded portions of fields may also have higher levels of corn foliage diseases.
Most of the control recommendations for minimizing losses due to corn foliage diseases are preventative measures such as planting resistant hybrids, rotating crops so the corn doesn't follow corn in the same field or tillage to reduce the amount of infected residue left on the soil surface. Several fungicides are labeled for use on corn to control foliage diseases. Please see the accompanying table of foliar fungicides labeled for use on corn.
Fields with high levels of various foliage diseases may also show higher levels of stalk rot this fall. As harvest approaches, check fields which have had foliage disease problems for stalk rot and try to harvest problem fields promptly.
Symptoms of Common Corn Foliage Diseases
Anthracnose (Colletotrichum graminicola)
Infection is most common on lower leaves of young plants but may occur on upper leaves of maturing plants too. Anthracnose lesions tend to be brown, spindle-shaped lesions with yellow to reddish-brown borders. Concentric rings or zones are sometimes apparent within the diseased areas. Stalk symptoms appear as black linear streaks on the surface of lower internodes late in the season.
Holcus Leaf Spot (Bacterial leaf spot)
Lesions usually are oval to rectangular in shape. Initially, they are dark-green and water soaked. Later they become dry and turn light brown with a reddish margin. The lesion resembles parchment paper. Holcus leaf spot may occur a few days after a rain storm but does not usually cause serious losses.
Common Rust (Puccinia sorghi)
Circular to elongate, golden-brown to reddish-brown pustules develop on both upper and lower leaf surfaces. As plants mature, the pustules become brownish-black. The pustules rupture, revealing powdery brown spores.
Southern Rust (Puccinia polysora)
Light, reddish-brown, circular to oval pustules develop primarily on the upper leaf surface. Eventually pustules rupture to reveal powdery spores. Later a brownish-black spore stage often forms in rings around the initial pustules.
Gray Leaf Spot (Cercospora zeae-maydis)
Lesions on maturing corn are pale brown to reddish-brown and blocky to rectangular in shape when compared to other corn leaf blights. The lesions typically are restricted by leaf veins giving the lesions parallel edges. Older lesions have a gray cast. Lesions may merge, resulting in large areas of dead leaf tissue. Lesions usually develop first on lower leaves but under favorable weather conditions, extensive leaf blighting over the entire plant may occur.
Northern Corn Leaf Blight (Exserohilum turcicum)
Long, elliptical, grayish-green or tan lesions ranging from 1.0-6.0 inches in length develop on the lower leaves. As the season progresses, nearly all leaves of a susceptible plant may be covered with lesions, giving this plant the appearance of having been injured by frost. During damp weather, dark olive-green to black spores may be produced across surface of lesions.
Southern Corn Leaf blight ( Bipolaris maydis)
Lesions are small, tan with buff to brown borders and somewhat elliptical in shape. Lesion appearance may vary greatly with genetic background of hybrids. Lesions may merge, blighting or killing large areas of leaf tissue.
Northern Corn Leaf Spot (Bipolaris zeicola)
Lesions are small, tan to reddish-brown and oval to circular in shape. Over time the lesions may become more tan to grayish-tan in color and be surrounded by a light to darkly pigmented border.
Eyespot (Kabatiella zeae)
Initial symptoms are small, translucent, circular to oval lesions with yellowish haloes. Later lesions develop tan to cream centers surrounded by a brown or purple ring with a narrow, yellow halo. Lesions may coalesce to from large areas of dead leaf tissue.
Laura Sweets
SweetsL@missouri.edu
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