By Ryan Flaming, County Extension Agent, Agriculture & Natural Resources
While scouting your corn fields after these rains are sure to be on the lookout for leaf diseases. If you don’t catch the diseases in time in can hurt your yields dramatically. These are some of the common corn leaf diseases to be watching for.
Common rust
It 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.
Gray leaf spot
Symptoms will start to 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.
Southern Rust
Its pustules look 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.
Anthracnose Leaf Blight
Its 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.
Goss’s Wilt
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.
Northern Corn Leaf Blight
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.
