Indiana Prairie Farmer publishes a column written by Tom Bechman with the help of CCAs for CCAs and their clients. With permission from Prairie Farmer we are posting these Soybean and Corn Pest Beat articles on the CCA website. Many thanks to the authors and the support of Indiana Prairie Farmer.
Corn Pest Beat Articles
Spot corn diseases early? Here’s what to do
The Indiana Certified Crop Adviser panel includes Gene Flaningam, Flaningam Ag Consulting, Vincennes; Carl Joern, Pioneer, Lafayette; Greg Kneubuhler, G&K Concepts, Fort Wayne; and Dan Quinn, Purdue University Extension corn specialist, West Lafayette.
I am leaning toward applying fungicide on all my corn this year, but the cash flow would work better if I don’t apply it unless it needs it. If I find disease symptoms early, say in late June or early July, do I spray then or wait? How do I know how much disease pressure is too much? When do I pull the trigger?
Flaningam: The optimal time to apply corn fungicide is tasseling (VT) to silking (R1). Start by selecting hybrids with good disease ratings. The two biggest disease concerns are southern rust and tar spot in Indiana. Scout fields pre-tassel to see what kind of disease infestations you might have. Look at the weather forecast to see if weather patterns are conducive to disease development. Review planting dates. Later-planted fields are usually most likely to be affected by diseases.
Joern: The right fungicide pass can pay, but more likely when disease, hybrid susceptibility and environment all align. The research is clear: the VT-R1 window is the most consistently profitable timing. That’s when yield potential is most sensitive, upper‑canopy leaves need protecting, and disease spread typically accelerates.
Finding disease early doesn’t automatically mean “spray now.” Most economically important diseases ramp up later, and fungicides only give you around 21 to 35 days of protection, depending on product. If you pull the trigger too early, you risk your residual running out right when disease pressure peaks.
Start scouting at late vegetative stages and scout weekly. If you’re seeing disease below the ear leaf, especially in continuous corn or susceptible hybrids, and conditions are favoring disease, you’re trending toward a VT-R1 spray. If the canopy is clean headed into tassel, you may be able to hold off.
Make the decision at VT-R1 unless disease is aggressively moving up the canopy or you’re dealing with a fast‑cycling disease like southern rust. Timing pays — not speed.
Kneubuhler: The highest and most consistent ROI fungicide timing in corn is VT-R1. The biggest mistake in fungicide decisions is spraying too early. What matters most is protecting the ear leaf and the leaves above it. If you spray too early, you may end up needing two applications, which can destroy return on investment.
Do not spray until you assess the following:
- where the disease is located
- how fast it’s progressing
- what the weather forecast shows
- susceptibility of the hybrid
Weather is a huge driver to disease pressure and intensity.
Tar spot may change the equation. If tar spot is observed early in the season, that would be a situation where spraying early would be warranted. Tar spot can spread explosively, and yield losses can be severe if you don’t stay ahead.
Quinn: If you find disease in the lower canopy and the crop is in late vegetative and early reproductive stages, then think about making a spray application, especially if you are experiencing cooler or higher humidity conditions. This suggests the disease will continue to persist and spread.
In most cases, a single application at the VT-R1 growth stage provides the most consistent and reliable response. However, if the disease is showing up later in the season and remains at low levels into later grain fill stages, then it likely will not be economical to spray.
The Crop Protection Network has a lot of valuable tools that include local data to help guide these decisions. The forecasting tools can help you monitor and make a more informed decision based on the location, crop growth stage and environmental risk for specific diseases.
