When is Fungicide Use in Corn Economical?

Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu

Each year around this time we receive questions about the chance that fungicide use in corn will pay. The answer depends on several factors including: 1) potential for disease (hybrid resistance level, environment, presence of disease), and 2) economics (grain price, yield potential, and the cost of fungicide application). This article is similar to one I wrote this time last year with some additional information and links.

1) Potential for disease. The first thing you need to do when deciding if you could benefit from a fungicide application to corn this year is to scout the field to determine how much disease you have and the identity of the disease in question. The most common diseases of potential concern in Delaware this year are Grey leaf spot and Northern corn leaf blight. These diseases originate from corn residue and can progress up the plant under favorable conditions (wet, overcast, moderate temperatures-see last week’s Weekly Crop Update for more information). Both diseases tend to be more problematic in no till fields of continuous corn. Under favorable conditions, these diseases can kill large areas of leaf tissue, placing carbohydrate stress on the plant and increasing the likelihood for yield losses and lodging. Anthracnose is less of a concern as a foliar disease after tassel because most hybrids produce chemicals in their foliage as they develop that inhibit the ability of the fungus to grow on/in newly developing tissue. As a result, anthracnose often is limited to lower foliage and does not progress very far up the plant to a significant degree. Anthracnose can cause a top dieback or stalk rot, but does so by infecting the plant through the roots. Consequently, foliar fungicides are not efficacious for suppressing anthracnose stalk rot or top dieback. For Grey leaf spot and Northern corn leaf blight, if you have greater than 5% severity on the ear leaf or two leaves leaves below the ear in 50% or more plants in the field just prior to or just after tassel then you might consider a fungicide application (Figure 1).

Figure 1. Examples of what 5% severity would look like for Grey leaf spot (top) and Northern corn leaf blight (bottom) of corn.

Other factors also impact the speed of disease development and disease severity. These include hybrid genetics and the environment. Hybrids with good ratings to Grey leaf spot or Northern corn leaf blight have sets of resistance genes that slow the development of lesions on leaves therefore the rate of disease spread. Susceptible hybrids allow the disease to progress at a normal rate and extensive lesions can develop, resulting in loss of photosynthetic area. Persistent wet, overcast conditions favor disease development. Extremely hot conditions (over 86°) greatly slow or cease development; however, it may continue once favorable conditions return.

A “worst case scenario” for a residue-borne disease such as Grey leaf spot is a no-till, irrigated field of late-planted continuous corn rated poorly for the disease in question. These fields are more likely to develop disease earlier in the season provide an environment more conducive for continued disease development. Table 1 provides some guidelines that may help you determine the risk level of your field based on hybrid resistance rating and irrigation practice. Here I use Grey leaf spot as an example.

Table 1. Hybrid resistance rating to Grey leaf spot, irrigation, and potential need for fungicide application. In this table disease is assumed to be present on 50% or more of plants at greater than 5% severity on the ear leaf or two leaves below the ear leaf.

Economics. With corn around $4.30 per bushel, a greater yield benefit is needed to cover application costs. Table 2 provides examples of the bushel returns you would need to cover fungicide treatment (applicator cost + product) at different grain prices.

Table 2. The required bushel/acre yield increases required to pay for various fungicide application costs at 5 different grain prices.

The likelihood that a fungicide will pay for itself is greatest in situations where disease potential is high, application costs are low, and grain prices are high. Research examining 187 separate corn fungicide studies from around the United States provides some insight into weather a fungicide will pay for itself in corn. Researchers estimated the chance that fungicide application costs ($16-40 / acre) would be covered by the yield return across a range of grain prices ($2-7 / bu). Results of the analysis indicated that in over 85% of the grain / application cost combinations, there was a greater than 50% chance that the application of a fungicide would not pay for itself if there was less than 5% disease severity on the ear leaf between R4 and R6. Conversely, 66% of the grain / application cost combinations payed when there was more than 5% disease severity on the ear leaf between R4 and R6. Therefore, the greatest chance for a grower to break even or profit from a fungicide is when the potential for disease reaching the ear leaf before R4 is high.

In addition, the study showed that although fungicide use in the absence of significant disease corn can be beneficial, responses are also highly variable from location to location and year to year. For example, fungicide applications reduced corn yields in 26-48% of the studies included in the metaanalysis. Since some companies suggest that products with group 11 fungicides improve stalk strength, individuals may apply fungicides as insurance against lodging. A discussion on the effects of fungicides on stalk strength in the absence of disease is too much for this week, but I encourage anyone interested in the subject to peruse an article entitled, “Are fungicides no longer for fungi? An analysis of foliar fungicide use in corn” by Kiersten Wise (Purdue University) and Daren Mueller (ISU) http://www.apsnet.org/publications/apsnetfeatures/Pages/fungicide.aspx. They have a nice section on foliar fungicides, stalk rots, and standability about three quarters through the article that provides a brief review of some experiments on the subject. Unfortunately there aren’t any exciting pictures in the article.

In sum: You need a greater bu/A yield increase this year to cover your application cost. You are more likely to recover this cost in disease favorable environments (no till, irrigated, corn after corn, history of GLS or other common residue-borne diseases) when susceptible hybrids are planted and disease is present on the ear leaf or 2 leaves below the ear leaf on 50% or more of plants just prior to or just after tasseling. In the future, we may want to look more closely at these relationships under Delaware conditions, particularly with such an abundance of products on the market and the amount of irrigated corn acres in the area.

Reference: P. A. Paul, L. V. Madden, C. A. Bradley, A. E. Robertson, G. P. Munkvold, G. Shaner, K. A. Wise, D. K. Malvick, T. W. Allen, A. Grybauskas, P. Vincelli, and P. Esker. 2011. Meta-Analysis of Yield Response of Hybrid Field Corn to Foliar Fungicides in the U.S. Corn Belt. Phytopathology 101:1122-1132.