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Is it time to apply fungicide? The answer varies by hybrid

To apply a fungicide on corn, or not to apply? That’s the question!

Is it time to apply fungicide? The answer varies by hybrid


To apply a fungicide on corn, or not to apply? That’s the question!

The Indiana Certified Crop Adviser panel searched for the answer from two different directions.

Panelists include Betsy Bower, agronomist, Ceres Solutions, Terre Haute; Steve Dlugosz, crops consultant, Harvestland Co-op, eastern Indiana; and Bryan Overstreet, Purdue University Extension ag educator in Jasper and Pulaski counties.

One hybrid I used is susceptible to gray leaf spot. It was a high yielder so I planted it anyway. Should I just schedule a fungicide application now, or wait and scout?

Overstreet: Scout before tasseling to see if gray leaf spot is present. If it needs it, the best time will be during tasseling or shortly afterward.

Ask yourself questions. Is the field in a corn-soybean rotation or corn after corn? Corn after corn will be more susceptible. What’s the weather forecast for the next two weeks? The disease needs high humidity, moisture and warm temperatures.

If you scout and don’t need to spray, you’ve saved money. If you have questions, call Kiersten Wise, Purdue Extension plant pathologist, at 765-496-3190.

Dlugosz: Warm, humid weather provides a favorable environment for rapid development of gray leaf spot, especially on susceptible hybrids. No-till corn may be particularly vulnerable. Schedul-
ing an application on high-yielding hybrids under these conditions can be profitable.

Bower: While it’s tempting to just schedule it for application, it’s best to scout first. Knowing it’s susceptible, it would be first on my list to scout. Check for long, rectangular, pale-brown or gray lesions on lower leaves first. If 50% of the plants have disease lesions on the third leaf below the ear leaf or higher prior to tasseling, and the hybrid is susceptible or moderately susceptible, consider application.

Resistant hybrids

My seed dealer claims every hybrid I planted has good resistance to most foliar diseases. Yet my chemical dealer is pushing fungicides. If I wait, I may not get product or airplane access. What should I do?

Dlugosz: Profitable returns from fungicide on corn are most common on highly susceptible hybrids. Responses are much less frequent on hybrids with good resistance to leaf diseases. Preferred application window is beginning of tassel to brown silk. Logistics of aerial applications can be complicated. Arranging the application early may be a good idea. You can always cancel later.

Overstreet: Results have been very inconsistent in showing higher results when disease pressure is not present. Many cases have shown no yield benefit. How much yield boost will it take to pay for application? At the current price of corn, it will take several bushels. I would look at previous crop, planting date, weather forecast and whether or not any disease is present.

Bower: On hybrids that have good resistance, the amount of yield protected if an outbreak occurs may be lower than cost of product and application.

If you have river bottoms, or corn after corn, and you want to see if it pays, try it. But be sure to leave an untreated check. Sometimes we ask applicators to spray perpendicular to planting direction leaving half to one-third unsprayed. There may be yield differences, and there may also be differences in stalk quality that could be important to you.


Scout and see: Note the gray leaf spot lesions on this corn leaf. Department of Plant Pathology Archive, North Carolina State University,

This article published in the July, 2010 edition of INDIANA PRAIRIE FARMER.

All rights reserved. Copyright Farm Progress Cos. 2010.

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