Warning for gray leaf spot on corn
Barton County Extension Agent
I was visiting with Doug Jardine, K-State extension plant pathologist, the other day and he commented about how bad gray leaf spot was on corn this year in some areas of the state and our area was one with potential for an outbreak. He said scouting of fields should be done prior to any fungicide applications. Many of the new hybrids have improved tolerance to gray leaf spot and may not require a fungicide application.
Most seed company catalogs will contain the gray leaf spot rating for a hybrid. Keep in mind, different companies may use different rating systems so you can only compare hybrids within a company, not between companies. For moderately susceptible hybrids, fungicide applications are appropriate, if lesions can be found on the ear leaf minus three on 50 percent or more of the leaves at tasseling.
The stage of maturity on the corn is all over the board due to spread out planting dates from the wet spring. A few acres were planted early, but most of it got in late. However, we have had a rapid recovery in heat units which has helped catch the corn up. Quite a bit of our corn is in the tasseling stage.
For intermediate hybrids, lesions need to reach the ear leaf minus two and, for moderately resistant hybrids, the ear leaf minus one.
If you have lesions on the ear leaf at tasseling, you need to spray regardless of the rated resistance.
Doug also told me that producers should keep in mind that there are many reports of crop injury when fungicides are applied prior to tasseling. Also, nearly all research shows that tasseling is the optimum time to apply a fungicide for maximum yield savings.
Reduced rate applications are strongly discouraged because of the possibility of fungicide resistance development.
Current registrations for many of the labeled products allow application beyond the full blister stage. This will allow for a later application if it is not clear yet that one is needed at tasseling.
The most commonly used systemic products contain chemicals from one or both of two major fungicide classes, strobilurins and triazoles. Strobilurins are primarily preventative, while triazoles have some curative properties. Strobilurins include Quadris and Headline. Triazoles include Tilt, Bumper, PropiMax and Foilcur. Then we have tank mixes of the two families which include Quilt, Quilt XL, and Stratego.
When making later applications, growers need to be sure to observe the pre-harvest intervals on the label.
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I want to address a couple of issues in this week’s editor’s note.