Late August is an exciting time for the Plant Health Challenge. With most of the corn in the area now in the dent stage, we are entering a great time to evaluate trials. As the crop approaches natural senescence, differences between fungicide programs can become much easier to identify.

Tar spot was first confirmed in our eastern territories of Webster, Adams, Franklin and Kearney counties on August 29, 2025, with just a few lesions observed in the lower canopy. This year, we are already seeing substantially heavier pressure. Tar spot has been a major focus of this year’s Plant Health Challenge, with many trials comparing one-pass and two-pass fungicide programs.

Recent scouting is beginning to show some encouraging visual differences. We are utilizing both a boots-on-the-ground approach and satellite and drone imagery to identify and document areas of interest. While tar spot lesions are being observed throughout many of these trials, differences in disease severity and senescence are becoming increasingly apparent. In some untreated areas, we are already seeing senescence around tar spot lesions at the ear leaf, while treated areas remain green deeper into the canopy.

Some of these differences are also beginning to appear in NDVI imagery, giving us another way to track how treatments influence plant health and senescence over time. These visual differences are encouraging, but the most important question still lies ahead: Will they translate into additional yield?

Over the next few weeks, we will continue scouting and documenting trials as the crop progresses toward maturity, collecting as much visual data as possible. NDVI imagery will help us track differences in senescence between treatments, and at harvest we will be able to compare those observations with yield data.

The next few weeks will be an interesting part of this year’s Plant Health Challenge as trials continue to separate and we get closer to putting numbers behind the differences we are seeing in the field.

 

What to Look For When Evaluating Your Corn Fungicide Application

By Dylan Catterton, Syngenta

Fungicide season has stretched on for weeks, and it isn’t over. Southern Rust has shown up across Kansas, Nebraska, and Iowa, and Tar Spot has been a steady presence in fields for months. As you walk fields to check how your application performed, here are the key factors worth keeping in mind.

Give It Time — Diseases Don’t Show Up Overnight

There’s a lag between when a corn plant is infected and when you can actually see lesions on the leaf. That lag varies by disease: Gray Leaf Spot takes 14 to 28 days to show up, Tar Spot 14 to 21 days, and Southern Rust just 6 to 12 days. That means if you wait to spray until you can see disease, more lesions are likely still coming — the infection was already there, just invisible. A triazole-based fungicide will stop infections from the last 2 to 4 days, but if disease is already entrenched in the leaf, it will keep developing into visible lesions for a while longer. No fungicide reverses damage or turns already-infected leaves green again.

Check Where the Disease Is on the Plant

Where a disease overwinters tells you a lot about whether it’s progressing or being held in check. Gray Leaf Spot and Tar Spot overwinter in last year’s residue, so they typically start at the bottom of the plant and move upward. Southern Rust is different — it blows in on the wind from tropical regions. In both cases, if you’re only seeing infection on the lower half of the plant, that’s usually a sign of good spray coverage, not disease escape. Coverage is generally considered sufficient if the top half of the plant — ear leaf and above — was protected. That upper canopy matters most, since it drives the bulk of photosynthesisand grain fill, and it’s also the hardest zone for a fungicide application to reach if coverage is marginal.

Planting Date Matters More Than You’d Think

Because planting was spread out this year, with some hail-driven replants mixed in, later-tasseling corn carries more risk from foliar disease. As inoculum builds in an area, disease pressure doesn’t rise steadily — it accelerates. A single Southern Rust pustule can produce roughly 700,000 spores a day for 18 days. Earlier-planted fields may be mature enough to avoid major losses, but they can still act as a rust “infection factory” for nearby fields that are in more vulnerable growth stages.

Watch Standability, Not Just Leaf Loss

Leaf area lost late in dent stage doesn’t always translate directly into yield loss — but if the plant can’t generate enough energy through photosynthesis, it will pull resources from the stalk to keep filling the ear. That weakens stalks and raises the risk of wind damage and lodging, which adds cost and time at harvest and leaves behind more volunteer corn to manage next year. This risk is compounded in fields that were already moisture-stressed earlier in the season, or in later-planted acres that will dry down in cooler fall temperatures with harvest likely delayed by logistics.

Bottom line: a lot of variables feed into how well a fungicide application performs. Understanding the “what” and the “why” behind what you’re seeing in the field will help you evaluate results accurately — and make better decisions for next season.