Corn Diseases Threaten Yields

Gray leaf spot is the number one disease in all corn production. Mature lesions tend to be rectangular, bordered by leaf veins.
Gray leaf spot is the number one disease in all corn production. Mature lesions tend to be rectangular, bordered by leaf veins. Photo courtesy of Daren Mueller, Iowa State University, Bugwood.org

To achieve maximum yield potential and excellent grain quality, corn has to survive a number of disease threats throughout the season, such as, grey leaf spot, northern corn leaf blight, and common and southern rust. Many disease pathogens cause blight, which is a specific response to infection. It is seen in the rapid chlorosis, browning and death of plant tissues. Diseases that exhibit this symptom are sometimes called “blights.”

Management for field corn, sweet corn and seed corn plant diseases is achieved through an integrated approach of best management practices and use of foliar fungicides and fungicide seed treatments.

Corn Leaf Diseases

Numerous corn diseases, including those caused by fungi and bacteria, exhibit symptoms in the leaves. Fungal and bacterial inoculum can overwinter in crop residue on the soil surface and/or be spread by wind and water. Certain other pathogens must have a living host to overwinter, and spores must be carried on wind currents for new infections. Foliar diseases that establish prior to tasseling, or become severe within two to three weeks after tasseling and pollination, can result in significant yield loss. Major diseases affecting the leaf include:

Gray leaf spot

Gray leaf spot is the number one disease in all corn production. The fungus survives the winter on residue from the prior corn crop, providing a primary source of the inoculum, particularly when growers plant continuous corn or use a no-till soybean/corn rotation. Spores from the fungus are splashed onto the lower leaves early in the season. When these resulting infections produce spores, the inoculum can be carried by air currents higher into the canopy or into neighboring fields.

Eyespot

Eyespot, a fungal disease that overwinters on surface corn debris, can be a severe problem. Early symptoms include small, light green, circular lesions with yellow halos. Lesions develop brown/purple rings as they age and may merge, destroying larger areas of leaf tissue.

Northern corn leaf blight

Similar to gray leaf spot, the fungal inoculum causing northern corn leaf blight survives in the residue in the soil and moves up through the canopy. This disease differs from gray leaf spot, however, in that it prefers cooler conditions, tending to strike the corn crop later in the season.

Common rust

Common rust is found everywhere in corn production. Both seed corn and sweet corn are very vulnerable to common rust, although the disease is not an economic problem in field corn. It tends to become less aggressive as the weather turns hot and dry.

Southern rust

Because of the changes in hybrid genetics over the last few years, southern rust has become more problematic. It has begun to spread all across the corn-growing region, starting in the South and being windblown to the North. Southern rust differs from common rust in that it thrives in hot conditions. As common rust starts backing off, southern rust explodes.

Southern corn leaf blight

Southern corn leaf blight lesions on leaves are seen at the mid-whorl stage through maturity. This fungal disease favors wet conditions and warm temperatures, as well as reduced tillage and continuous-corn production.

Goss’s leaf blight

As the bacteria that causes this blight overwinters primarily in crop residue, it favors continuous-corn production and reduced tillage. In addition to the leaf blight phase, this disease can also have a systemic wilt phase, which leads to a slimy stalk rot.

Anthracnose

This fungal disease, which attacks the plant at various stages of growth, can appear early in the season as a leaf blight. Later it can attack the stalks above the ear leaf leading to “top die-back,” or lower on the stalk soon after tasseling leading to reduced plant productivity and lodging.

Stewart’s wilt

Stewart’s wilt, a bacterial disease, appears on the leaves as pale green, linear lesions that turn yellow and then brown. The margins of each lesion are wavy or irregular. The disease overwinters in its host and vector, the corn flea beetle.

Common smut

Although primarily a disease of the stalks or ears, symptoms of common smut also appear on tassels and leaves. Tumor-like galls form on actively growing tissue and are white or greenish-white. The interior darkens and turns into a powdery, black clump of spores. Galls on leaves normally remain small, dry and hard.

Stalk Diseases

Infection by most stalk rot organisms, both fungal and bacterial, can occur early in the season. Infection, however, becomes obvious during grain fill. Stalk rots can cause significant yield loss when the disease causes plants to deteriorate prematurely, resulting in poor ear fill or light test weight grain. Lodging can also cause heavy yield losses.

Many stalk rot diseases first infect the roots and then progress into the lower stalk. These diseases plug the vascular tissue in the stalk, limiting the uptake of water and nutrients from the soil, causing a significant impact on overall yield and test weight. They are also critical because they impact the strength and vitality of the stalk; the weakened stalk can tip over and cause lodging. The occurrence and severity of stalk rot diseases depend on many factors, such as plant stress.

Anthracnose is a major stalk rot disease, causing problems in many types of corn and notable for attacking the plant at various stages of growth. It appears early in the season as a leaf blight. Later in the season it is visible in stalk dieback; the top parts of the stalk often snap off because the vascular tissue is plugged and the stalk is rotting at the top. It enters through the roots and invades the lower section of the plant, but it can also invade the stalk itself.

Other significant stalk rots include Fusarium stalk rot, Gibberella stalk rot, Diplodia stalk rot, charcoal rot and bacterial stalk rot.

Seed and Seedling Diseases

Several different soilborne or seedborne fungi in corn can cause seed and seedling rots, also called pre- and postemergence damping-off. These early diseases can rot seed, reduce emergence, slow growth and stunt plants, thereby reducing yield.

Rhizoctonia

Rhizoctonia root rot can discolor and decay the outer layer of the main root and stem at the soil line and below. Rainfall, followed by cool and then warm, humid weather, is conducive to development of this disease.


Managing Corn Diseases

A well-thought-out disease-management program, including best management practices, proper seed protection and selection, and fungicide applications using multiple modes of action, should be implemented to sustainably manage corn diseases .

A seed treatment provides great protection against early fungal diseases, especially in cool and damp spring conditions, and helps plant seedlings get off to a vigorous start. Bayer’s seed treatments result in more uniformity down the row and consistency in the development of the plants.  Also, timely fungicide applications, including foliar fungicides, help protect corn plants from fungal diseases throughout the season. For farmers, a fungicide decision is a matter of considering his production needs, risk management, timing of his various agronomic practices, and an eye to commodity prices.

Stratego YLD® fungicide from Crop Science controls a broad spectrum of diseases; it combines the latest in triazole technology with a powerful, complementary strobilurin chemistry. This versatile fungicide can be applied early season and/or at tassel. The early-season application delays the onset of infection in the middle of the canopy and fights infections of the stalk. The tassel application protects the photosynthetic engine, the ear leaf and the surrounding leaves. Both applications are especially important for growers who are pushing for high yield production and/or farming in an area with a disease history.

Yield increases over untreated corn have been consistently demonstrated, with a nearly 7-bushel-per-acre increase with use of an early-season application and a nearly 12-bushel increase with use of a tassel application. Growers have been shown to gain 15 bushels when using both the early and tassel applications.  

Keep in mind that an early-season fungicide application provides important protection and typically adds a yield bump to your crop. For cost efficiency, fungicides can be tankmixed with herbicides and applied in one trip across the field.

For more information on corn disease management solutions from Bayer, contact your local Crop Science US representative or visit bayercropscience.us/crops/corn.  

Before purchasing seed, selecting a seed treatment or applying any fungicide, please read the entire label for the best possible results and to confirm that the product is effective on the disease you need to control. Every product is not suitable for every situation, and correct application technique will ensure the best results.

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