Some parts of Kansas have had recurring problems with gray leaf spot in corn and this year is no exception, according to Doug Jardine, a Kansas State University Research and Extension plant pathologist.
"Recent surveying has indicated that it will be a very good year for gray leaf spot in many of the areas where it has traditionally been a problem," says Jardine. "In the Kansas River Valley, for instance, gray leaf spot has already reached the ear leaf minus two in some continuous-corn fields where the tassel was just beginning to exert itself. On the other hand, southeast Kansas appears to be free of the disease at this time."
Jardine says 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," he says. "For moderately susceptible hybrids, fungicide applications are appropriate if lesions can be found on the ear leaf minus three on 50% or more of the leaves at tasseling."
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," Jardine says.
He says 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," he adds.
Current registrations for many of the labeled products allow application beyond the full blister stage, the plant pathologist says. This will allow for a later application if it is not clear yet that one is needed at tasseling.
When making later applications, growers need to be sure to observe the preharvest intervals on the label.