Foliar Fungicides for Corn – Yes or No?

Nathan Kleczewski, Extension Specialist – Plant Pathology;; @Delmarplantdoc

We are entering that time of year when growers start to think about fungicides for corn. Depending on who you speak to you will hear that they range from being 100% necessary to 100% unneeded. In reality, it’s likely somewhere in between. Before I get too far into what to consider, let’s go over how foliar diseases can impact a corn plant.

Foliar pathogens of corn tend to fall into two groups. One group produces toxins that kill plant tissues and the pathogen feeds off of these decaying or dead tissues to grow and reproduce. An example of such a disease is Gray leaf spot. The second group of foliar pathogens produce “straws” that allow the pathogen to siphon nutrients from living host tissues. These pathogens need the plant to be alive in order to grow and survive. Examples of this type of pathogen include the rusts.

When either type of pathogen affects the plant, particularly the ear leaf after tasseling, it uses carbohydrates that the plant needs for filling the ear. The ear leaf and leaf below provide the majority of carbohydrates for ear fill, and this is why they are the most important tissues to protect and focus on. When carbohydrates are limited, the plant can start to draw nutrients from its reserves in the roots and stalk to support ear fill. Consequently, the roots and stalk may not have sufficient nutrients and energy to survive, and these tissues may weaken. The end result is typically reduced yields, and potentially greater chance of lodging later in the season. In sum: if the ear leaf is significantly impacted by foliar disease before black layer, there is a chance you may see yield loss and standability issues later in the season. Remember- I am saying that there is a chance. I am not saying it is a certainty.

Gray leaf spot.

Northern corn leaf blight.

That being said, foliar fungicides will benefit you and your operation if you are at high risk for commonly occurring foliar diseases. What are the most commonly occurring diseases that you will see in Delaware and Maryland? 1) Gray leaf spot and; 2) Northern corn leaf blight. These are residue-borne diseases that need persistent, wet conditions to infect foliage. Diseases such as common and southern rust can occur very sporadically, typically not until very late in the season, and are not considered significant, yield-limiting diseases in this region.

Now that you know what diseases may impact your corn, how do you go about determining the likelihood that a foliar fungicide can help you? Paul Vincelli, from the University of Kentucky likes to use the idea of a risk ladder. The higher up you are on the ladder, the greater your risk of falling to disease-related problems, and the more likely you will see a benefit from a fungicide application. What are the ways to move up the risk ladder? Here is my list of the major factors influencing foliar fungicide effectiveness, in order of importance.

The higher up you are on the corn disease ladder the greater the risk you are for falling to disease related issues.

1) Hybrid resistance to commonly occurring foliar diseases. Is your hybrid rated good to excellent for Gray leaf spot or Northern corn leaf blight? If yes, then the risk of developing these diseases, regardless of other conditions, is greatly reduced. If your hybrid is rated poor to fair for these diseases, then, if the conditions favoring disease occur, a fungicide application could be beneficial.

2) Residue and rotation. Are you planting into ground that was in corn last season? Are you planting no-till? Increased amounts of corn residue provides more material for the Gray leaf spot and Northern corn leaf blight pathogens to overwinter, grow, and potentially infect your corn under the right conditions. If you rotate to soybean or other crops such as vegetables, or till your ground, the amount of these pathogens available to infect your field is greatly reduced.

3) Irrigation. Are you heavily irrigating your corn? The longer the amount of leaf wetness, the greater opportunity for foliar diseases to infect the plant. Dryland corn only receives water with rain and therefore, risk is related to weather patterns.

4) Standability of your corn. Did you purchase a hybrid with good to excellent stay green or standability ratings? If yes, then the potential for late season lodging, even if facilitated by foliar disease, is greatly reduced.

After going over these factors, where are you on the ladder? If you are high up and at risk of falling, then what can we say about the foliar fungicides? First- when should the fungicide be applied and how many applications are needed? For a fungicide application to be the most effective, applications should occur between VT and R1. If disease is not present at this time the application can be delayed as far as R3. Research on multiple fungicide applications has indicated that growers will see the greatest return for their investment by applying a single fungicide application at this timing. Unless you are growing for the yield contest, multiple applications are not required and are not likely to be economically beneficial.

In addition, there are many products to choose from. For an unbiased rating of the most commonly used foliar fungicides for corn click here. I’m not going to discuss which products are the best, rates, etc. However, I will say that you should be certain that you are seeing a return on your applications. Are you being told that a new product is, “the best”? Are you hearing the word, “guarantee”? Prove it to yourself. After all, it’s your field, right? How can you do this? Leave an untreated section or strip of the field to allow yourself to compare yields and standability at the end of the year. Ensure that the section can be harvested and that it is representative of the field. Do not use field edges. By doing this you will have an idea of if the application had an impact in that field. Simply comparing fields is not appropriate. Why is this? Because each field will differ in conditions that will impact disease and yield. As such, any differences you see are confounded with a multitude of other factors that may impact yield. Untreated areas or strips can be related to a specific field and therefore allow you to make more informed decisions pertaining to treatment effectiveness.

Remember- foliar fungicides are a helpful tool for producing corn, but are not always required. Look at your risk ladder and if you decide you want to make an application, save yourself some guesswork and leave an untreated area of the field to assess later in the season. They say a picture is worth a thousand words, right? The same can be said for conducting a simple, on farm trial.

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