No. 16/July 11, 1997
Conditions Favorable for Gray Leaf Spot
With the high temperatures, frequent rainfall, and high humidity, conditions are favorable for gray leaf spot (GLS) to appear on corn leaves. Howard Brown, agronomist for Pioneer Hi-Bred, reported finding lesions on lower leaves in fields in west-central Illinois. The disease is present but does not represent any real threat to crop development at this time; however, it is increasing in severity across the Midwest, due in part to the adoption of reduced and no-till practices. This pathogen survives readily in corn debris and sporulates profusely in the early spring if weather conditions are favorable. The disease is most severe in continuous no-till corn and can cause extensive damage in reduced tillage fields if crop rotation is not practiced.
Lesions are identified by their rectangular or blocky appearance on susceptible corn plants. Lesions are pale brown or gray to tan in color and are 1/4 to 2 inches in length. They are restricted by the veins and usually have blunt or squared-off ends. In susceptible hybrids, lesions may coalesce, causing extensive tissue necrosis.
GLS typically appears on the lower leaves because the spores are being either windblown or rain-splashed from previous crop debris. Extensive blighting may result, followed by death of the plant and stalk breakage or lodging. If favorable environmental conditions occur, GLS may kill entire fields before they mature.
GLS is favored by warm, humid conditions and frequent rainfall. Fungal spores can survive at humidities as low as 60 percent, but infection and colonization of the host does not occur unless relative humidities climb above 85 percent. This pathogen has a long latent period when no symptoms are visible. This period may last from 2 to 4 weeks. Thus, once initial symptoms are evident, the disease severity may already have reached the epidemic point.
Control of gray leaf spot should begin with identification of potential problem fields and the selection of resistant or tolerant hybrids for these fields. Because GLS is favored by high humidities, only tolerant or resistant hybrids should be planted in these fields. River bottom fields, for example, are typically humid and offer the most favorable environment for GLS infection. Crop rotation is also important for GLS control. This pathogen cannot survive for extended periods without a host plant. Thus, rotating soybeans or another nonhost crop helps to reduce the inoculum level.
Plowing heavily infected fields also reduces carry-over inoculum levels. Once buried, the fungus cannot produce spores and infect the corn crop. However, care should be taken with plowing, especially with regard to slope of fields and erosion considerations. Separation of fields can be a minor, although important, method of reducing infections by GLS. Because this pathogen is primarily wind dispersed, corn crops should not be planted adjacent to a field with high corn residues if GLS was a problem in that field the past season. If winds blow across the residues, spores may be transported to the new corn crop and early infections begun.
Field scouting for GLS should begin at least 2 weeks before tasseling and continue until at least 2 to 3 weeks past tasseling. If lesions are at or above the ear leaf, a fungicide application of Tilt may significantly improve performance. It is difficult to determine an exact threshold for leaf damage from GLS because hybrids vary in their response to infection. Some hybrids can tolerant rather extensive damage before yield losses appear while others will react to lesser amounts of damage. Various estimates place the leaf area damage in the range of 10 to 25 percent on the bottom four leaves before economic losses can be expected to occur. This allows for a bit of risk adversity on the part of the producer when determining the need for fungicide applications. Decisions to apply fungicides should be made with the performance of the hybrid in mind, as well as the 30-day weather forecast. If past experience indicates that this hybrid is sensitive and the forecast is for continued favorable weather (warm, high humidity, rainfall), then a yield benefit can be expected from a Tilt application. Expected response to Tilt applications can range from 10 to 30 bushels per acre, depending on the factors mentioned. However, the key to economical benefit for any fungicide is scouting and an application made based upon the presence of the pathogen. The critical period for protecting the plant is usually 2 weeks before to 2 weeks after tasseling. However, if leaf blights are present earlier and appear to be moving upward on the plant, earlier fungicide applications may be needed. This practice is not recommended because it usually means that you will need other applications made later, when they would be far more effective.
H. Walker Kirby, Extension Plant Pathology (217)333-8414