Robertson delivers disease outlook for Iowa row crops
By Jean Caspers-Simmet
simmet@agrinews.com
Date Modified: 07/22/2010 9:17 AM
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NASHUA, Iowa —Alison Robertson, Iowa State University Extension plant pathologist, said Ohio and Indiana colleagues are getting questions from corn growers about adding fungicide to their glyphosate.
"I have a hard time understanding why you would do that," Robertson told farmers at the recent field days at the Nashua and Kanawha research farms. "There's little disease pressure at that point, and if you do have a problem later, you'd have to reapply fungicide because the effective period of a fungicide is 14 to 21 days."
She and her colleagues in Wisconsin and Indiana conducted trials in 2009 applying Headline to corn at V6, R1 and V6 plus R1. There were no obvious benefits to the applications.
Most corn and soybean pathogens survive in infested surface residue for up to 12 months, Robertson said. Common corn pathogens are eyespot, which likes cool weather and frequent rains; gray leaf spot, which likes warm, humid weather; and northern corn leaf blight, which favors frequent rains and overcast conditions.
Common soybean pathogens are brown leaf spot, favored by frequent rains; Cercospora leaf blight, favored by warm humid weather; and frogeye leaf spot, favored by warm humid weather.
Planting resistant varieties is the most effective way to manage corn and soybean diseases. Crop rotations and tillage can help for diseases that overwinter in residue. Fungicides can also be used to manage diseases.
Robertson said the warm, wet start to the growing season has been favorable for brown spot development in the lower soybean canopy. If current weather conditions continue, brown spot could move up into the mid-canopy of soybean plants, and impact yield. A foliar fungicide application at R3 could be a good decision.
Before applying a fungicide to soybeans consider economics, Robertson said. Scout fields to determine disease pressure.
Before applying a fungicide to corn, consider the price of corn, price of product plus application and drying costs, Robertson said. Consider hybrid susceptibility and the disease history of the field. Scout fields to determine disease pressure. Look on plant leaves below the ear leaf. If there are several spots, and a susceptible hybrid was planted, farmers may want to consider spraying a fungicide.
Robertson said foliar fungicide trials at Nashua from 2007 through 2009 showed that yield response was greatest when a fungicide was applied the last week of July/first week of August. In 2008 and 2009, this corresponded to growth stage R1, but in 2007, the corn tasseled early, and the R2 application resulted in the greatest yield response.