by Boyd Padgett, Ph.D. and Clayton Hollier, Ph.D.
I have received numerous calls from producers, consultants, and agents reporting diseases in corn. These reports are earlier compared to previous years, and disease management strategies will change based on this fact. The diseases most prevalent are southern rust, common rust, and northern corn leaf blight. The LSU AgCenter DOES NOT recommend an automatic fungicide application to corn. HOWEVER, when disease epidemics are progressing in young corn (tassel or earlier) a fungicide will be needed to slow epidemics and protect yield and quality.
Ideally, fungicides should be applied prior to disease onset, but realistically, fungicides are usually applied at or just after onset. Therefore, individuals should make efforts to detect and treat diseases as early as possible to prevent losses to yield and quality. When it is determined an application is needed, a premix fungicide will offer wide spectrum activity (examples are: Headline AMP, Stratego YLD, Quilt, and Quilt Xcel). Follow label instructions for application timings, rates, etc. In most cases, a single application at tassel is justified when disease is present and active. The decision to apply a fungicide should be made on a field by field basis. The remainder of this newsletter will address disease identification and management considerations.
Diseases can be found in Louisiana corn fields every year, but their impact on grain quality and yield is dependent on several variables. Therefore, the decision to apply a fungicide should be based on a solid understanding of disease initiation and development. Diseases commonly found in Louisiana corn include common and southern rusts, smut, and northern corn leaf blight. Other diseases that occur less frequently are ear and stalk rots, gray leaf spot, and southern corn leaf blight. While these diseases rarely develop to statewide damaging levels in most years, disease incidence and severity in individual fields may warrant a fungicide application. However, before applying a fungicide, several factors need to be considered. These include: disease identification, environmental conditions favoring disease development, and the relationship between disease severity and yield loss.
Identification and Development
Common rust usually can be found every year in Louisiana (figure 1). This disease is caused by the fungus Puccinia sorghi. In some fields signs of this disease can be present early in the growing season. Common rust is usually the first disease present in Louisiana corn, but subsides when temperatures exceed 77oF. Initial infections occur from wind-blown spores from corn-producing areas in tropical and sub-tropical regions of the world. This disease usually does not cause yield loss in Louisiana corn.
Conditions favoring development:
Moisture period: 6 hours of leaf wetness or high relative humidity
This disease can be found in fields prior to tasseling. Sporulation on the leaf surface can occur within 7 days after infection. Pustules are elongated, ragged looking and occur on the upper and lower leaf surfaces. Spores with pustules are cinnamon-brown in color. In some cases, pustules occur in bands because of infections that occurred while the leaf was in the whorl.
Southern rust, caused by the fungus Puccinia polysora, is also present in Louisiana corn (figure 2). Similar to common rust, initial infections are caused by wind-blown spores. This is a warmer-season rust compared to common rust and usually occurs late season and does not have adequate time to impact yield. However, this rust is very aggressive and if disease epidemics initiate early (prior to or at tasseling), yields could be reduced.
Conditions favoring development
Moisture: High relative humidity or abundant rainfall
Southern rust produces small circular to oval pustules and contain orange to light brown spores. The spores are lighter in color when compared to spores associated with common rust. Pustule size is usually smaller and less ragged looking than those produced by the common rust pathogen. Pustules are more abundant on the upper leaf surface and can also be found on the leaf sheath when disease is severe.
Northern Corn Leaf Blight
Northern corn leaf blight is caused by the fungus Exserohilum turcicum (figures 3&4). This disease was present at damaging levels in some fields during the 2010 growing season. The disease can be found in Louisiana corn fields during mid-season (tasseling/flowering) and, in some cases, can cause yield loss. The fungus can survive on infected corn debris left on the soil surface from the previous growing season. Therefore, the risk to this disease increases in fields where reduced-tillage practices are used and corn is planted continuously. Corn debris from the previous season provides inoculum for disease initiation and establishment. Spores produced on this debris are disseminated by wind and rain splash infecting the current crop. Subsequent infection results from spores produced within lesions on the current crop. There are several races of this pathogen. Therefore, the effectiveness of genetic resistance may vary depending on the races present in a particular field.
Conditions favoring development
Moisture: 6-18 hours
Lesions of Northern corn leaf blight usually begin in the lower canopy and progress upward. Lesions begin as small elliptical or spindle shaped lesions. Mature lesions can be six inches in length and about ½ to 1 inch wide. The lesions are grayish green in color.
Less Common Disease and Abnormalities
Gray Leaf Spot
Gray leaf spot is caused by the fungus Cercospora zeae-maydis. The fungus can overwinter on infected corn debris from the previous season. Therefore, risk to disease is increased when corn is continuously cropped and reduced tillage allows debris to overwinter.
Conditions favoring development
Moisture: Repeated moisture over 11 or more hours OR high relative humidity
(95% or more)
Initial lesions are rectangular, small, necrotic, with yellow halos. As lesions mature, they expand and turn gray where the fungus may sporulate on the underside of the leaf.
This disease is caused by the fungus Ustilago madis and is generally not thought to impact yield. This disease is usually present at very low levels in every corn field, and is most severe when actively growing tissue of young corn is wounded. The fungus overwinters on infected corn debris from the previous growing season or in the soil (for many years). The fungus is NOT seedborne, as is the case with some smuts in other crops.
Symptoms can occur on foliage and ears and are very evident. Individual kernels enlarge and are silvery gray in color. Diseased kernels can be cut in half to reveal black sooty spores.
Purple leaf sheath
Each year this abnormality can be found in some corn fields within the state. THIS IS NOT A DISEASE. While fungi and bacteria are associated with this condition, this is not harmful to the plant. The purple discoloration on the stalk and leaf sheath results from colonized (fungi and bacteria) pollen that is lodged between the sheath and stalk.
Risk and Management
Risk to disease is influenced by several factors including: genetic resistance, tillage practices, planting date, and environmental conditions. Plant debris left on the soil surface (no-till or reduced-tillage production systems) harbors disease pathogens which increase the risk to disease. Later planted corn can also heighten risk to southern rust and possibly common rust. A favorable environment will always increase risk; therefore, it is important to know what conditions favor disease development.
The first line of defense for managing corn diseases should be selecting a disease-resistant variety. When resistance is not available, a fungicide may be needed. However, when disease is not present a fungicide is not necessary. Another factor to consider is when disease epidemics initiate relative to crop growth stage. The potential for yield loss is high when disease develops prior to tasseling and conditions remain favorable for development during the growing season. When disease initiates after tasseling, the potential for disease loss decreases. Therefore, a fungicide may not be needed even in the presence of disease.
The relationship of yield and defoliation can be found in table 1 adapted from the National Crop Insurance Service ‘Corn Loss Instruction’.