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2013 a good corn fungicide year?

The greater likelihood that foliar diseases could knock down your corn yield potential through the rest of this summer means it's as good a year as any to consider applying fungicide, says one plant pathologist.

A lot of this year's corn crop was planted late into wet, cool soils. That means it's more susceptible to damage from diseases caused by bacteria and fungus, like Gray leaf spot, Northern corn leaf blight and Goss's wilt. That's mainly because grain fill -- a critical time for disease infection -- will be later than normal, says Iowa State University plant pathologist Alison Robertson.

"In the past 2 growing seasons, most of the corn in Iowa has been flowering by mid-July. This year, it will be another 10 days or more before widespread tasseling occurs in the state. When diseases start during early grain fill, this increases the risk of reduced yield," she says in a university report. "Research has shown that greater yield responses due to a fungicide application consistently occur in the presence of disease."

Target any fungicide applications on fields that are likely to see disease damage at this stage, around R5, Robertson adds. But, that susceptibility's not always the most obvious thing to nail down. Consider these 3 factors, she recommends:

  • Hybrid: Hybrids vary in their tolerance to various diseases. “Race horse” type hybrids are often susceptible to one or more diseases. Fields planted to susceptible hybrids may require a fungicide application to protect yield.

  • Presence of disease: Gray leaf spot and northern leaf blight often develop in the lower canopy and then move up the plant. Common and southern rust are more often found in the mid to upper canopy. Presence of leaf spots in the canopy means the weather has been favorable for disease development and inoculum is present in the field.

  • Weather conditions during grainfill: For disease to happen, or continue to develop, certain environmental conditions need to occur.

There are 2 caveats to putting down a fungicide as the corn crop reaches grain fill. First, make sure you're falling within the desired pre-harvest interval (PHI) for the product you're applying. Some PHIs can stretch to beyond 6 weeks, Robertson says.

You can also actually make your crop more susceptible to disease damage -- albeit from something completely different -- by applying a fungicide at the wrong time. That makes it critical to avoid jumping the gun with your application.

"Pre-tassel applications of fungicides may result in arrested ear syndrome. Research at Purdue University showed spray additives (NIS surfactants and crop oil) to pesticides applied between V10 and VT increased the risk of this disorder," Robertson says. "Wet spring conditions across Iowa have resulted in uneven stands that range widely in crop growth stage. Try to wait to apply a fungicide until the field has completely tasseled. Some fungicides do not require the addition of a surfactant – check the label, however when fungicides are applied aerially, crop oil is used to ensure the fungicide product does not evaporate and is deposited within the canopy."

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