Sometimes it simply pays to spray

Corn Corner: When should you pull the trigger on fungicides? Experts weigh in. 
May 30, 2020
To save money, I don’t want to spray fungicides on corn unless it’s a must. What should I look for and when?
The Indiana certified crop advisers panel answering this question includes Betsy Bower, agronomist, Ceres Solutions, Terre Haute; Jesse Grogan, regional manager, AgReliant Genetics LLC, Lafayette; and Stan Miles, agronomist, A&L Great Lakes Labs, Fort Wayne.
Bower: Be prepared to keep up with what’s going on agronomically. The Purdue Pest and Crop Newsletter is a great source of weekly agronomic info. There are also phone apps called the Purdue Corn Field Scout and Soybean Field Scout. They cost a few dollars but are great assets.
Scout at least once per week starting around V10. Yield-impacting diseases you can see at V10 to V12 include physoderma, northern corn leaf blight and gray leaf spot. For physoderma, an early application of fungicide when needed can improve yield and standability through harvest. Each disease has an economic threshold.
Two later-season diseases that can highly affect corn are tar spot and southern rust. Both diseases can be extremely detrimental to yield. Some hybrids are more susceptible.
Research would indicate the best return on investment with many corn diseases is an application of fungicide around R1 to R2, soon after tasseling till brown silk. Exceptions are early infections of physoderma and very late infections of tar spot and southern rust on susceptible hybrids.
Most fungicides have mixed modes of action — a good thing. While most are pretty good on northern corn leaf blight and gray leaf spot, there are differences on tar spot and southern rust. Consult your local crops
professional for guidance.
Grogan: Hybrid selection, tillage system and crop rotation are the best routes to avoid fungicide rescue decisions. Choose hybrids that have at least a moderate level of tolerance or resistance to key foliar diseases. Factor in yield, standability and plant health. Risk factors such as tillage system, irrigation and weather should be considered.
Disease pathogens like gray leaf spot are associated with crop residue on the soil surface, enabling early onset. Applying fungicides at V6 as a preventative treatment isn’t consistent or efficient. Best timing for a one-time fungicide application is tassel or flowering. Scout corn in the late-whorl stage. Look for lesions or rust pustules for southern corn rust. If lesions or pustules are on the ear leaf, a fungicide is likely beneficial when wet and warm weather follow.
Choose products according to the target. This fungicide efficacy table is a good resource. Return on investment should be considered. Best results are in fields with high yield potential.
Miles: If the hybrid at the top of the yield chart caught your eye but has a lower disease tolerance, scout these fields and monitor for disease progression and possible fungicide treatments.
Gray leaf spot normally thrives on susceptible hybrids when temperature and humidity are high. When the temperature is in the high 90s and grass is too wet to mow at 11:30 a.m., scout gray leaf spot. When lesions are at the ear leaf or above prior to flowering, the potential for yield loss is present.
Northern corn leaf blight prefers cooler temperatures through grain fill. Southern rust must be transported into Indiana on strong thunderstorms that blow spores up from the Gulf of Mexico.
Careful scouting, attention to weather and environmental conditions, and a strong knowledge of your hybrid’s disease tolerance ratings will help you make the best economic decision.