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Corn Leaf Diseases and Fungicides for Their Management

July 5, 2002
As we near July 4, much of the corn crop in Illinois is growing rapidly because of the recent warm weather. Much of the crop is doing relatively well in spite of the excessive rain and late planting this spring. Now it's time to consider corn leaf diseases, and initial reports of leaf diseases have begun to come in. We cannot predict if leaf diseases will be a significant problem this year. Incidence and severity will be determined to a great extent by rainfall, humidity, and temperature over the next 2 months, but it is time to begin scouting. Corn leaf diseases can cause greater yield losses if infection occurs early in the growth stages of the plant. Because much corn in Illinois is behind normal development stages at this date because of late planting and poor growing conditions in May, this year the potential for increased risk of losses from foliar diseases is greater.

Although many corn leaf diseases occur in Illinois, some common diseases in one or more areas of the state are rust, gray leaf spot, eyespot, northern leaf spot, and northern corn leaf blight. This article will provide a brief description of the first three of these diseases; factors contributing to their development; and management tactics, including fungicide options for susceptible inbreds.

Common rust (caused by Puccinia sorghi) was a significant problem in parts of Illinois in 2000 but not in 2001. Occurrence of rust depends on cool temperatures and timely introduction of rust spores via wind and rainstorms from southern areas (this pathogen does not overwinter in Illinois), usually from mid-June to mid-July. Infection occurs primarily during conditions of high humidity, with over 6 hours of moisture on the leaves and temperatures between 60 deg F and 76 deg F. Young leaves are more readily infected than older mature leaves. According to some reports, 10% total leaf area infected can result in yield reductions of 3 to 8%. Common rust is managed with resistant hybrids and/or appropriate applications of fungicides.

Gray leaf spot (caused by Cercospora zea-maydis) may be the most damaging corn leaf disease across the Midwest. It was found in southern Illinois last week. Gray leaf spot (GLS) caused minimal yield losses across Illinois in 2001 but was widely found near the end of the season when it was too late for it to cause significant yield losses. This may indicate that there may be much GLS inoculum in corn residue in many fields this season. Spore production usually begins on infested residue in June, and if spores are spread to leaves on the new crop a 1- to 2-week lag period may occur before lesions become visible. GLS requires high humidity (>95%) for 24 hours, warm temperatures (75 deg F to 85 deg F), and susceptible inbreds or hybrids to occur at significant levels. Symptoms first appear on lower leaves, and initial lesions appear as small, tan spots, 1 to 3 millimeters long, irregular to rectangular in shape. Lesions turn gray to tan and are rectangular in shape (parallel to leaf veins) as they mature. GLS is managed by avoiding fields that were planted with corn last year, using resistant hybrids, and using timely applications of fungicides to susceptible inbreds.

Eyespot (cause by the fungus Kabatiella zeae) is reported to be a sporatic problem in the northern part of Illinois but can be severe at times. Like GLS, the pathogen overwinters on infested corn debris. Initial symptoms are small (1 to 4 millimeters in diameter), light green, watersoaked, circular lesions with yellow halos. Lesions develop brown/purple rings around them as they age and may coalesce to kill larger patches of leaf tissue. Eyespot is managed with the same tactics as for the management of GLS.

This season another fungicide product option is available for control of corn leaf diseases. In addition to the commonly used Tilt, Quadris, Bravo, and mancozeb-containing products such as Penncozeb, Stratego is now labeled for corn. Stratego from Bayer Corporation received a label in June 2002 for control of leaf diseases of field corn, popcorn, and seed corn. Stratego contains two active ingredients--trifloxystrobin (similar to Quadris) and propiconazole (same as Tilt). Preliminary testing of Stratego in 2001 in southern Illinois suggested that it can be an effective product, along with Quadris and Tilt, for control of gray leaf spot. It also is effective against many corn leaf diseases. Other fungicides, including the products previously named, also can be effective for control of corn leaf diseases. Always read and follow product label recommendations when using any fungicide.

The key time for scouting and application of fungicides is typically about 2 weeks before tasseling and about 1 week after tasseling when lesions have appeared and weather conditions favor disease development. Professor Don White, corn pathologist at the University of Illinois, recommends that dent corn hybrids should not be sprayed with fungicides under most conditions because the odds are low for a favorable return on the fungicide investment. However, seed corn inbreds are a different story, and fungicide applications will frequently result in favorable returns from increased seed yields and quality.

For additional information on control of corn leaf diseases, refer to the University of Illinois Field Crop Scouting Manual and the 2002 Illinois Agricultural Pest Management Handbook. Both books are available through your local University of Illinois Extension office.--Dean Malvick

Author: Dean Malvick

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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