Wallaces Farmer

What you should be thinking before deciding on foliar fungicides

Genetic resistance is your first line of defense before foliar fungicides.

May 10, 2024

3 Min Read
tar spot on corn leaf
TAR SPOT TROUBLES: Tar spot of corn is a relatively new disease to the U.S. and is recognized as raised, black, irregular spots scattered across corn leaves. Courtesy of Iowa State University

by Alison Robertson

Twenty years ago, when I started at Iowa State University, foliar fungicides were rarely used on hybrid corn. Grain prices, new fungicides labeled for use on corn, increased foliar diseases and marketing fungicide use on hybrid corn have meant that fungicides are now a common input for many farmers.

So, what should you think about now before deciding on foliar fungicide applications this growing season?

  1. Disease management starts with genetic resistance. Fungicides should never be used as the first line of defense against a disease. Selecting hybrids that have resistance to disease should be the foundation of all disease management plans. If you have concerns about a particular disease, check the seed catalog or speak with your seed salesperson to identify the best hybrid for your area.

  2. Choice of product. The prime purpose of a fungicide is to control fungal or fungal-like diseases, such as tar spot, gray leaf spot, northern corn leaf blight and southern rust. Numerous fungicide products available may make deciding on which product to spray difficult. The publication “Fungicide Efficacy for the Control of Corn Diseases,” which is available for download from the Crop Protection Network (cropprotectionnetwork.com), can help with that decision. Most known fungicide products are listed along with their efficacy against a disease. The publication is based on land-grant university research data and is updated annually.

  3. When to spray? The goal for protecting yield should be to keep the ear leaf and canopy above as healthy as possible during grain fill. The grain fill period (from silking to black layer) lasts 55 to 65 days. Most fungicides are effective at reducing disease for 21 to 35 days. If you spray too early and disease pressure is high, a second application may be required to protect the crop later into grain fill. More than 15 years of data from multiple locations across Iowa suggests that applying a fungicide when tassels and silks are present (growth stage VT/R1) through to when silks begin to dry up and turn brown, reduces disease best in most years. National Predictive Modeling Tool Initiative is developing tools to predict disease and thereby help with fungicide decision-making. The Tarspotter app was developed with data collected by the NPMTI. Farmers can input the GPS coordinates of their fields into the app and use it to predict the best timing for a fungicide application to control tar spot.

  4. Fungicide resistance. As with all pesticides, developing resistance in the target organism is a concern. Similar to herbicides, fungicides are grouped according to their mode of action. Resistance to fungicide mode of action Groups 11 (QoI strobilurins) and 3 (DMI triazoles) have been reported in soybean and wheat in the United States. For example, QoI strobilurin fungicides have no efficacy against the fungus that causes frogeye leaf spot on soybean.

Perhaps more important, however, is that resistance to azole antifungal agents has been reported in fungal pathogens of humans here in the United States and throughout the world. DMI triazole fungicides used in agriculture have an identical mode of action as azole antifungal agents used for human health. There is growing evidence that the use of DMI triazoles in agriculture is causing resistance to azoles in human pathogens. The Centers for Disease Control and Prevention estimates that more than 35,000 people die each year in the United States because of antimicrobial-resistant infections.

Robertson is an Iowa State University Extension field crops pathologist.

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