indiana Prairie Farmer Logo

Make decision to control gray leaf spot

Corn Pest Beat: Disease presence does not always warrant spraying fungicide.

May 30, 2024

3 Min Read
A hand pointing to gray leaf spot lesions on a corn leaf
SCOUT AND DECIDE: Are these two gray leaf spot lesions below the ear leaf enough to justify a fungicide application? The CCA panel says it likely depends upon expected conditions and timing. Tom J. Bechman

I am on the fence about spraying fungicides. All my hybrids are somewhat resistant to gray leaf spot, but I am finding two or three little lesions regularly on the leaf below the ear leaf already. Should I spray? If so, when? What?

The Indiana certified crops adviser panel answering this question includes Danny Greene, Greene Crop Consulting, Franklin; Abby Horlacher, Nickel Plate Consulting, Frankfort; Brian Mitchem, Farmer 1st Agronomy Consulting Services, Decatur; and Dan Quinn, Purdue Extension corn specialist.

Greene: Some considerations include potential revenue that the crop could lose if not sprayed, extended weather outlook and your risk tolerance. Chances are, you have asked this question before. Work with your agronomist to set up some strip comparisons in fields to assess a fungicide’s return in this year’s conditions. If replications and location comparisons are put together, you can have more confidence in your plan for 2025.

The most recommended growth stage for fungicide application for gray leaf spot is VT to R1. However, if the forecast for the next few weeks is dry and lesions aren’t spreading, application may be delayed. Most fungicide applications likely also help control several other fungal diseases. However, southern rust and tar spot sometimes become problems after the R1 application runs its course.

To determine what fungicide may best fit your need, check out CPN-2011-W, a multistate guide for fungicide efficacy in corn.

Horlacher: If strictly spraying corn for disease present, two to three lesions doesn’t constitute a green light to spray. When making a fungicide spray decision based off disease, look at lesions you have, disease forecasting from universities and weather you are experiencing. If it stays wet and humid and universities forecast more disease coming, I would apply fungicide to protect your crop. If we’re going into a dry spell, hold off and do not spray a fungicide.

Mitchem: Specifically with gray leaf spot, it takes considerable infection to merit spraying. Leaf diseases such as northern corn leaf blight, southern rust and tar spot are much more impactful. As noted, most corn hybrids have a solid defense package for gray leaf spot. Rarely do we battle just one foliar disease. Consider all potential pathogens.

If you spray, the target remains at R1 for most return on investment. There are many excellent fungicides from various companies. We don’t have enough space to discuss the merits of each individual product. Consult with a local CCA for the products of choice for your specific situation.

Quinn: What time of the growing season are you observing these lesions? If you find gray leaf spot lesions approaching the ear leaf or on the ear leaf in the first two weeks before or after tassel emergence, a fungicide is likely needed. A fungicide application at R1 to R2 in corn is often the most consistent timing for controlling multiple foliar diseases and improving yield. However, if these lesions are found later at R4 to R5 or after black layer, a fungicide is likely not going to be beneficial.

As for what to spray, there are many different products that help control gray leaf spot. See the fungicide efficacy table from the Crop Protection Network. It is a great resource to examine available products and determine their ability in controlling specific foliar diseases. Use this resource to make your fungicide selection.

Read more about:

Fungicide
Subscribe to receive top agriculture news
Be informed daily with these free e-newsletters

You May Also Like