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Agronomy Day 2007

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Foliar Fungicides for Corn and Soybean....the New Norm or Just a Fad?

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Carl A. Bradley Carl A. Bradley
Assistant Professor of Plant Pathology
and Extension Specialist
Department of Crop Sciences
Tel: 217-244-7415
E-mail: carlbrad@illinois.edu

The use of foliar fungicides on corn increased dramatically in the 2007 growing season. Factors such as commodity market price, increased corn-on-corn production, and fungicide marketing programs have helped drive fungicide use on corn in 2007. Fungicide products registered for use on corn, such as “Headline”, “Quadris”, “Quilt”, and “Stratego”, contain an active ingredient in the “strobilurin” fungicide class (i.e. azoxystrobin, pyraclostrobin, or trifloxystrobin). Strobilurin fungicides are relatively new to the commercial corn industry and have efficacy against a wide range of diseases, such as gray leaf spot and northern corn leaf blight. Unfortunately, over-use of these strobilurin fungicides can lead to increased risk of fungicide resistance. One of the ways to help reduce the risk of fungicide resistance is to use good IPM practices that rely on things like scouting observations and weather to help make fungicide application decisions for disease control. Hybrid susceptibility may play a key role in determining when a foliar fungicide application will be an economic benefit. A foliar fungicide trial on corn in Urbana, IL in 2006 over ten corn hybrids varying in their susceptibility to gray leaf spot, showed that a 6 fl oz rate of a strobilurin fungicide provided anywhere from a 0.5 to 12 bu/A increase over the untreated check, depending on the hybrid.

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“Effect of a strobilurin fungicide on yield of ten different corn hybrids. From a 2006 trial conducted in Urbana, IL (data courtesy Dr. W. Pedersen, Univ. IL).”

Similar to fungicides available on corn, soybean fungicide products used in the midwest typically contain a strobilurin as a solo or combination active ingredient. The strobilurin fungicides control a wide range of soybean diseases as well, which include frogeye leaf spot, soybean rust, and anthracnose. For most of these diseases (especially soybean rust), the strobilurin fungicides work best when applied preventatively, and have little effect on diseases after the infection process has begun. This is true for two reasons: 1) the strobilurin fungicides are powerful at inhibiting fungal spore germination, which means they prevent the spore from developing a germination tube that eventually would penetrate into the leaf cuticle; and 2) the strobilurin fungicides are generally attracted to and concentrated in the outer cuticle of the plant tissue, therefore, strobilurins are not effective once the plant pathogenic fungus has penetrated the cuticle and is growing throughout the inside of a leaf. “Triazole” fungicides have been the most widely-used products for soybean rust control in South America and the southern U.S. The triazole fungicides can be used preventatively, but also have some limited-curative properties that help it control soybean rust. In the U.S., many of the triazole fungicides available for use on soybean to control rust have section 18 emergency exemptions. These special exemptions allow these fungicides to be used on soybean only for the control of rust, and not for control of any other diseases. Regional data from the midwestern U.S. have shown that economic benefits of fungicide applications on soybean occurred approximately 25% of the time when disease pressure was absent or low; however, when soybean rust has been present at economic levels in the southern U.S., fungicide applications have generally been beneficial. A network of “sentinel plots” have been established across the eastern U.S. to monitor for the presence of soybean rust. The data from these sentinel plots are used to track soybean rust in the U.S., and can be viewed on a map at the url: www.sbrusa.net. Using the soybean rust map on this website can help growers make fungicide application decisions based on the presence or risk of soybean rust moving into an area.