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Should you spray? Guidelines for economical corn fungicide decisions

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Column by Matt Adams, Hardin County Extension Service agent

With today’s corn prices and this year’s awesome yield potential, producers are looking for every way possible to increase yield. From new, more accurate planting and harvesting equipment, to stacked trait seed corn, to irrigation systems, it seems anything that has a possibility of increasing gross income goes.

Another way of possibly increasing yield is application of fungicide. This practice definitely has potential to increase yield, but yield response can be highly variable and the cost of applying fungicide can be high. While we need to try and produce every bushel of corn possible, we need to be conscious of our decisions, and be sure the net return will outweigh the costs.

In the instance of corn fungicides, there are a lot of claims that routine, scheduled applications of fungicide can make corn more drought, cold and stress tolerant, and increase yields up to 25 bushels per acre. Sounds great, right? Well, let’s look at the science behind the claims and come up with the most practical approach.

First, let’s look at how these fungicides work. Most fungicides pushed for routine application are Strobilurin fungicides. These include any systemic fungicides or fungicide mixtures containing an active ingredient ending in “strobin.” These usually are the most effective fungicides available for corn. Contact fungicides simply do not the efficacy and duration of protection for adequate control of foliar diseases in corn.

Speaking of foliar disease, what are the main diseases targeted by Strobilurin fungicides? The two most common diseases in Kentucky are grey leaf apot and northern corn blight. Both of these diseases can overwinter in infected corn residue and typically are the two diseases targeted by fungicide applications.

The next question is, what does a fungicide application cost? Most university assumptions say a fungicide application costs somewhere between $20 and $25 per acre. Assuming $5-$6 per bushel corn, this equates to roughly four bushels per acre. While some may think four bushels per acre is a small risk to take, ask yourself, what if there is no response? If that turns out to be the case, then four bushels per acre at $5 per bushel sounds pretty expensive. Over a large number of acres, that could equate to enough to make a payment on a piece of equipment, or even pay an employee’s salary for the year.

So what is the probability that spraying in the presence of minimal disease pressure will pay off? As an example, let’s look at a 2009 university trial. In this particular trial, Headline Fungicide was applied at a rate of 6 ounces an acre at tasseling. In the trial, there were 130 cases where disease pressure was less than 5 percent. In these cases, the average yield increase was around four bushels per acre, just enough to cover the cost of the fungicide application. Only 37 cases showed disease pressure above 5 percent, and in these cases, the average yield increase was 9.6 bushels per acre, which translates into a pretty good return on investment.

Another claimed advantage is drought tolerance. The drought tolerance results in a “leaf greening” effect, which is evident in the lower leaves of the corn plant after it enters the “R” stages of development. This greening effect doesn’t always mean an increase in yield, and actually can cause slower dry down, raising moisture levels at harvest. In a University of Illinois trial, treated corn was approximately 2 percent higher in moisture content at harvest time. This extra moisture means the corn must be dried more and can as much as double drying costs in some instances, raising the four bushel per acre breakeven even higher.

So, am I suggesting producers forget about fungicides all together in corn? Absolutely not. In some cases, fungicide applications can prove very profitable, especially in the presence of disease pressure, and disease develops during weather patterns like we’ve experienced this year. Farmers should scout their fields routinely and be on the lookout for disease pressure, particularly grey leaf spot and northern corn blight. As I stated before, these diseases overwinter in infected corn residue, so disease pressure may be higher in corn-after-corn situations, or in fields where disease pressure has been high in the past. These fields will have a higher probability of benefiting from a fungicide application. Other diseases, such as southern corn rust, do not overwinter in the state and must be carried in by wind from areas in the southeast. Southern corn rust is being monitored all across the Southeast and Midwest, so if it has potential to become a problem, local producers probably will be warned it is coming, giving them time to prepare. The monitoring system is updated weekly and can be viewed online at http://sba. ipmpipe.org/cgi-bin/sbr/public.cgi?host=Corn&pest=southern_corn_rust.

The most economical approach to fungicide application seems to be to scout for disease, determine the level of disease pressure, and spray when disease pressure reaches a threshold. Sure, we need to produce every bushel possible, but we should be aware of the costs of our practices so we can achieve maximum net profit.

Matt Adams is a Hardin County Extension Service agent for agriculture and natural resources.