There are several leaf diseases that can infect corn in Kansas in any given year. They can all be controlled with some combination of hybrid selection, tillage management, crop rotation, planting dates, or foliar fungicides.
The primary corn leaf diseases of concern in Kansas are:
Anthracnose leaf blight
Anthracnose leaf blight. Photo courtesy of Ohio State University, ohioline.osu.edu/ac-fact/0022.html
Symptoms are tan, irregular-shaped lesions on the lower leaves as early as V3 to V4. Lesions may reach a half-inch in length, with a red, reddish brown, or yellow orange border.
Anthracnose is most common in fields with old corn debris present. High temperatures and cloudy, rainy weather favor infection.
Resistant hybrids can be used to control this disease, but producers should be sure that the hybrid is resistant to anthracnose leaf blight, not just anthracnose stalk rot, since the two types of resistance are different. Producers can also help reduce this disease by using rotation or tillage to eliminate crop debris. Use of foliar fungicides to control early anthracnose has not been demonstrated to be profitable.
Common rust on corn. Photo courtesy of Iowa State University, www.ent.iastate.edu/imagegal/plantpath/corn/comrust/0796.37comrust.html
This disease is typically less serious in Kansas than the other leaf diseases. Symptoms are small, round to elongated pustules that start out golden brown then turn darker later in the season. Common rust pustules commonly form on both sides of the leaf and are sparser than those of southern rust.
This disease can occur wherever corn is grown. Infection is favored by moderate temperatures (60 to 77 degrees) and high relative humidity (greater than 95 percent for at least six hours).
Common rust is easily controlled by using resistant hybrids. Fungicides are not recommended for this disease alone since common rust causes only minimal yield loss.
Goss’s bacterial wilt
Goss's wilt. Photo courtesy of University of Nebraska, pdc.unl.edu/agriculturecrops/corn/gosswilt
This disease is caused by a bacterial, not a fungal, infection. Symptoms are gray to light yellow stripes with wavy margins that follow the leaf veins. Within these lesions, dark green to black, water-soaked spots that take on the appearance of freckles usually appear and are an excellent diagnostic symptom.
This disease occurs primarily in northwest Kansas, northeast Colorado, and southwest Nebraska. It can be controlled with resistant hybrids and crop rotation.
Gray leaf spot
Gray leaf spot on corn. Photo by Doug Jardine, K-State Research and Extension
Symptoms develop on the lowest leaves first and progress upward. The first symptoms are tiny lesions surrounded by a yellow halo. These eventually elongate into pale brown or gray rectangular lesions ranging from less than an inch to two inches in size. The entire leaves may become blighted.
Gray leaf spot survives in infested plant debris on the soil surface. In Kansas, initial infections occur in late June and early July. Cloudy weather accompanied by prolonged periods of leaf wetness and high humidity favor disease development. Severe damage often occurs in low spots or in fields bordered by trees or streams where air circulation is poor.
To control gray leaf spot, producers can use a crop rotation that is long enough to eliminate corn debris. Producers can also till under the old corn debris. There are many hybrids available with at least partial resistance. Producers can also use foliar fungicides when the economic threshold is exceeded. Application of a fungicide prior to full tasseling is not recommended as crop damage can occur prior to this stage of development.
Northern corn leaf blight
Northern corn leaf blight. Photo courtesy of Iowa State University, www.ent.iastate.edu/imagegal/plantpath/corn/northleafblight/ncorn_leaf_blight_0796_02.html
Symptoms are gray, elongated lesions 1 to 6 inches long. The lesions appear on the oldest leaves first, and progress upward. Lesions may become tan as they mature.
Northern corn leaf blight is most common in continuous corn where crop debris remains on the surface. Conditions that favor infection are temperatures of 65 to 80 degrees with extended periods of dew.
There are several hybrids with resistance to northern corn leaf blight. Producers can also help reduce this disease by using rotation or tillage to eliminate crop debris.
Southern rust on corn. Photo courtesy of University of Nebraska, http://cropwatch.unl.edu/archives/2006/Crop18/socornrust.htm
Southern rust pustules looks similar to common rust, but there are usually a lot more of them and they occur only on the upper leaf surfaces. This often gives the upper leaves a dusty appearance.
Southern rust does not overwinter in Kansas. Spores blow up from southern production areas in mid- to late-July. Warm, humid weather favors infection.
Resistant hybrids are the best choice for management. If susceptible hybrids are planted late, and disease conditions are favorable, applications of a systemic foliar fungicide may be warranted.
The following are leaf diseases that can occur in certain situations:
Continuous corn, with residue on the surface: All
Continuous corn, no residue on the surface: Common rust, southern rust
Rotated corn: Common rust, southern rust
The following lists leaf diseases according to the time of year they typically occur in Kansas:
1. (Earliest in the season) Anthracnose leaf blight
2. Gray leaf spot
3. Common rust
4. Northern corn leaf blight
5. Goss’s wilt
6. (Latest in the season) Southern rust
The following lists leaf diseases according to how commonly they occur in Kansas:
1. (Most common) Common rust
2. Gray leaf spot
3. Southern rust
4. Anthracnose leaf blight
5. Goss’s wilt
6. (Least common) Northern corn leaf blight
The following lists corn leaf diseases in order of the potential yield loss they typically cause under moderate to severe infections:
1. (Most severe yield loss) Gray leaf spot
2. Southern rust
3. Goss’s wilt
4. Anthracnose leaf blight
5. Northern corn leaf blight
6. (Least severe yield loss) Common rust
Doug Jardine, Extension Plant Pathologist