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Issue No. 15, Article 5/July 3, 2008

Making Profitable Fungicide Applications in Corn

In 2007, a record number of corn acres were sprayed with foliar fungicides in Illinois and other midwestern states. A similar trend is expected for 2008. But are all these applications necessary? The simple answer is no, because a foliar fungicide application to corn will not always provide a yield (or economic) benefit. Data summaries from both university and commercial sources all agree on this point--that foliar fungicides did not provide a benefit on corn every single time they were applied in 2007.

Summary of results from university corn fungicide trials conducted in 2007 in 12 different states and Ontario.

The real question that should be asked is under what circumstances a foliar fungicide applied to corn will be profitable. Because the main purpose of using a fungicide is to control a disease, the best answer is this: when foliar diseases are at a high enough level to cause economic losses. The problem lies in the fact that once you know a foliar disease is significant enough to cause economic loss, there is little you can do about it. So what option does that leave (besides just spraying everything prophylactically)?

Determining a cornfield's risk of developing a severe foliar disease problem is something that can be done to help make a fungicide application decision. Certain production practices and factors can play a role in a cornfield's foliar disease risk. When many of these risk factors are present, the likelihood of greater disease pressure increases (and thus does the likelihood of profitable fungicide application). I'll discuss some of those risk factors here.

Previous crop and tillage practice. When corn was the previous crop and a substantial amount of residue is left on the soil surface, the risk of foliar disease increases. Many of the fungal pathogens that cause foliar diseases on corn survive in corn debris.

Planting date. Research conducted by Dr. Gary Munkvold at Iowa State University and by Ron Hines at the University of Illinois Dixon Springs Agricultural Center has indicated that late-planted corn is more at risk to gray leaf spot than early-planted corn.

Hybrid susceptibility. Most hybrids are rated for their susceptibility to diseases like gray leaf spot and northern corn leaf blight. When hybrids with greater susceptibility to these diseases are planted, the risk of the diseases increases. Results from university trials conducted in 2007 in 12 different states and Ontario indicated that hybrids with a "fair to poor" rating for gray leaf spot resistance had a 6 bu/A increase when a foliar fungicide was applied, compared to a 4 bu/A increase when a foliar fungicide was applied to hybrids with a "good to excellent" rating for gray leaf spot resistance.

Results from multi-university trials in 2007, showing that hybrids with less disease resistance had a better yield response to foliar fungicides.

Weather and environment. High relative humidity and moisture are important for the development of foliar diseases on corn. In corn fungicide trials conducted in 2007 by the University of Illinois (Carl Bradley and Emerson Nafziger) and Southern Illinois University (Bryan Young), the greatest yield increases occurred in the regions of the state that had the most rainfall in July. The average yield increase observed in fungicide-treated plots was 5 bu/A in northern Illinois compared to only 1 and 3 bu/A in southern and central Illinois, respectively.

The average yield increase from foliar fungicides was greater in northern Illinois than in southern and central Illinois in 2007. This was due to greater rainfall in northern Illinois and greater disease pressure

Disease observations. Scouting fields prior to tassel emergence may give an indication of potential disease pressure. The earlier that some diseases are apparent, the greater the risk of losing yield. This is especially true for rust. No hard-and-fast economic thresholds are available for foliar corn diseases, but scouting can give an indication of how quickly diseases are building on the lower leaves. Be aware of the following fungicide guidelines based on scouting observations (adapted from G. Munkvold, Iowa State University):

  • For susceptible or moderately susceptible hybrids, a fungicide application should be considered if the disease is present on the 3rd leaf below the ear or higher on 50% of the plants before tasseling.
  • For intermediate hybrids, fungicide application should be considered if conditions and factors are favorable for disease and if disease is present on the 3rd leaf below the ear or higher on 50% of the plants before tasseling.
  • For resistant hybrids, a fungicide application is generally not recommended, but field scouting is still important.

What about hail-damaged corn? The most recent questions I have received about foliar fungicides on corn deal with whether to spray a hail-damaged cornfield with a fungicide. The fungi that cause foliar diseases of corn do not need wounds to cause infection on the leaves, nor is there any evidence that wounds even favor infection by these pathogens. Wounds from hail may provide access for bacterial pathogens to gain entry and cause infections, but a fungicide will not control bacterial diseases anyway. These wounds may also provide an entry for the fungus that causes common smut, but common smut is not controlled by foliar fungicides either. If the risk factors discussed above are present, then it may make sense to apply a fungicide to this field, but the hail damage need not play a role in the decision (except for yield potential, perhaps).

A simulated hail-fungicide trial was conducted at Urbana in 2007, with corn plants being damaged with a string trimmer just before tasseling to simulate hail damage. Some plots were left undamaged as well. The fungicides Headline, Quadris, and Quilt were applied to the plots and compared to an untreated check. When the data were statistically analyzed, fungicides did not significantly improve yield compared to the untreated check in the "hail-damaged" plots or the nondamaged plots. The simulated hail damage alone did decrease yield by approximately 30 bu/A compared to the nondamaged plots, however.

Results from a simulated hail damage x foliar fungicide trial on corn at Urbana in 2007.

What about yield increases despite lack of disease pressure? As a plant pathologist, this is a question that I hate to tackle, because it is actually an issue of plant physiology that has nothing to do with plant pathology (diseases). That being said, it is true that some fungicide classes (such as strobilurin fungicides--the active ingredients in Headline and Quadris and one of the active ingredients in Stratego and Quilt) can have other impacts on plants besides disease control. One of the most visual results of the strobilurin fungicides that can happen to corn plants is a "greening effect." This effect does not automatically add yield to corn plants, however. In fact, the effect was observed in some of my research plots in 2007, in which no differences in yield occurred between fungicide-treated and untreated plots. In my opinion, the primary reason for applying a foliar fungicide should be related to controlling diseases. If other benefits occur, that is great, but that should not be the primary reason to apply the fungicide.

Greening effect of Headline fungicide observed on the lower leaves. Picture was taken in 2007 in a fungicide research trial at the University of Illinois Dixon Springs Agricultural Center; no significant differences occurred for yield between fungicide-treated and nontreated plots in that trial.

Overall, foliar fungicides are a great disease management tool to have available for corn production. With escalating commodity prices and the demand for greater yields, we are all looking for ways to bump corn yields; however, there are no silver bullets. Foliar fungicides can help increase production and profits, if they are used appropriately.--Carl A. Bradley

Carl A. Bradley

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