Applying Fungicides to Corn May Not Pay

ISU ran on-farm tests in 2007 to see whether applying fungicides mid-season to corn would boost yield enough to increase profit.

Published on: Dec 26, 2007

An estimated 3 million acres of corn in Iowa were sprayed mid-season with fungicides (strobilurin or a strobilurin/triazole combination) in 2007. Farmers who sprayed say the reason they did is because of the high price for corn, the potential to control diseases and the possibility of getting an improved yield from the "plant health" benefits.

"Until this year, fungicide applications to production cornfields were rarely practiced in Iowa because they were not profitable. In addition, many of the hybrids grown today have good overall tolerance to foliar (leaf) diseases," points out Roger Elmore, an ISU Extension agronomist.

Three ISU Extension specialists--Roger Elmore, Alison Robertson and Lori Abendroth-developed a research program to examine whether using mid-season fungicides on corn would increase yield enough to increase profits.

High corn prices prompt fungicide use

"Producers are looking to maximize corn yields today more than previous years because of higher grain prices," says Elmore, ISU Extension corn specialist. "Some of the management practices that were not economical at one time may now be, considering the new prices. Fungicide application to field corn is one of those management tactics that needs reexamined given the new economics and the increased interest in these products. We wanted to evaluate the yield responsiveness associated with an application of a mid-season foliar fungicide on farmer fields across the state."

In the 2007 growing season, ISU Extension personnel and Corn and Soybean Initiative partners worked together to collect data to:

  • evaluate the control of gray leaf spot, common rust, and stalk rot from a mid-season foliar fungicide application
  • assess the grain yield response of corn to foliar fungicide application
  • determine if foliar fungicide applications are profitable
  • identify agronomic characteristics (such as hybrid and previous crop) that are important in determining when to spray fields.

Need at least a 6 bushel yield increase

"The foliar fungicide applications significantly reduced both common rust and gray leaf spot severity," says Robertson, an ISU Extension plant pathologist. "In addition, stalk rot severity also was lower in the sprayed treatments. However, the mean yield response to the foliar fungicide application was below a break even yield response of six bushels per acre. This breakeven (6 bushels per acre) was calculated based on the cost of a fungicide application which ranged from $18 to $24 across the state, and a corn price of $3.75 per bushel."

Research summarized over 26 of the 35 locations to date identified an average yield increase of 3.3 bushels per acre when a fungicide was applied at tasseling or silking compared to an untreated control in Iowa. Yet 3.3 bushels per acre is below the yield necessary to cover fungicide and application costs. Only 27% of the time was a fungicide application profitable. Fungicides decreased foliar disease severity and stalk rot severity but did not always result in a positive or profitable yield response. Data from the remaining nine locations will be added when they are available.

Should you spray corn in 2008?

The 2008 growing season is several months away, but already farmers are making decisions regarding the purchase of fungicides. However, a decision to apply a fungicide mid-season to hybrid corn should be based on IPM management practices, says Robertson. Foliar fungicide is simply a management tool that is useful only in conditions where disease pressure would be expected to significantly reduce yield potential.

"Remember, any management tool is only going to protect yield when there are factors that would reduce a field's potential yield, says Abendroth, an ISU Extension corn specialist who works with Elmore. "In this case, if there is significant disease pressure then a fungicide application at tasseling or silking could potentially protect yield during the grain fill period," she says.

"Yet if disease pressure is light, as was seen across most of Iowa in 2007, we would not expect an economical yield increase simply because the crop is not experiencing significant stress and can therefore proceed through grain fill without a problem," adds Abendroth.

Consider these factors before deciding

The three ISU specialists say you should consider the following factors before spraying a fungicide in 2008: hybrid susceptibility, disease pressure at VT growth stage of the corn, weather conditions at VT and during grain fill, previous crop and the amount of crop residue present in the field.

In general, a fungicide application is not recommended on resistant or moderately resistant hybrids. On susceptible, moderately susceptible or intermediate hybrids, the decision to apply a fungicide is based on the level of disease pressure at the time near application.

Protecting the corn crop from a stressful growing environment is critical in attaining high harvestable yields, notes Robertson. Any type of stress that is left unmanaged could reduce either the maximum yield potential or the harvestable yield, depending on when stress occurs in the growing season.

To understand how yield is determined and which yield components could potentially be changed in response to a mid-season fungicide application, the three ISU specialists say you should read "Corn growth and yield formation in light of fungicide applications at tasseling." It is available online at  

A more complete report of the research is available in the Dec. 10, 2007 issue of ISU's Extension Integrated Crop Management Newsletter, which is available online