I apologize; I realize this article is late. And, if I'm being honest, my team had a really nice article on corn fungicide efficacy put together that I scrapped and pivoted from when I realized the extent of fungicides going out due to fears over southern rust and tar spot.

The typical recommended application window falls between tasseling (VT/R1) and the milk stage (R3). For those who are still deciding whether to spray, let's look at what's happening in the fields right now regarding our major fungal disease threats so you can make the best call on those acres.

Southern Rust:

• So far in 2026, the only U.S. confirmations are in southern Texas.
• The fungus that causes Southern Rust cannot survive the winter in most of the U.S. Spores must migrate up from the South each year.
• Currently, there's not enough threat of southern rust to warrant an application.
• This is something to monitor in our late-planted corn, especially if warm temperatures persist.
• You can monitor real-time movement of Southern Rust here: https://corn.ipmpipe.org/southerncornrust/

Tar Spot:

• We confirmed the disease in Missouri in June, which is like the last four years.
• Each case that I have been involved with has shown very low levels of infection in the lower canopy.
• The current heat should slow fungal growth and disease spread.
• However, once temperatures begin to cool down again to 24-hour averages of 65°F to 73°F, the disease is likely to start taking off.

We've also had multiple reports of Gray Leaf Spot and Northern Corn Leaf Blight, which were mostly restricted to the lower canopy but are additional diseases to monitor as they approach the ear leaf.

Timing, ROI, and Protecting the Ear Leaf:

I realize that fungicide application timing is not always an easy, clear decision. In general, we want to protect the ear leaf between the tasseling (VT) growth stage and the milk growth stage (R3). A fungicide application before there is a threat to the ear leaf or outside of the VT to R3 window is much less likely to yield a positive return on investment (ROI). If you want to analyze the data for your specific field, there is a free fungicide ROI calculator available on the Crop Protection Network: https://connect.doit.wisc.edu/cpn-roi-tools/. You can adjust disease pressure alongside your specific yield goals, corn prices, and fungicide application costs.

Adding to the challenge in corn is that complexity of airplane availability. If you are on the fence about spraying, waiting may be an option that wasn't available just a few years ago.

In collaboration with the Mizzou Weed Science team, we have found that drone applications provide similar, and in some instances, better disease suppression and yield protection compared to traditional aerial applications. You can view those preliminary findings here: https://weedscience.missouri.edu/slideshows/UAV_fungicide_trials_2025.pdf.

We'll be discussing corn fungicide applications along with many other topics at the upcoming Mizzou Crop & Pest Field Day on July 8th. Registration is free and the deadline is July 3rd. Click here for more details: https://extension.missouri.edu/news/free-crop-pest-management-field-day-ag-innovation-showcase-is-july-8