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Corn+Soybean Digest

Corn growers: Tips To Decide If A Fungicide Application Is Warranted

Now is the time of year when corn growers need to decide if a fungicide should be applied and Kiersten Wise, a Purdue University expert has a few guidelines to help in the decision making process.

"Deciding whether to apply a fungicide is not a straightforward question," says Wise, Purdue Extension field crops disease specialist. "There are several things that must be taken into consideration."

University research from across the Midwest shows consistent yield advantages from a fungicide application occur when there is a significant amount of disease in a field, Wise says.

"This is the time to get out there and scout and determine the amount of disease in the field," she says. "This will really help you decide if a fungicide application is justified."

Wise recommends farmers take into account hybrid susceptibility, the level of disease in the field, previous crop and cropping system, late planting and if weather conditions are favorable for disease development.

Hybrids vary in their susceptibility to foliar disease of corn, and hybrids susceptible to diseases such as gray leaf spot are at a greater risk of disease development than hybrids with moderate or high levels of resistance, she says.

"As far as limiting yield potential, gray leaf spot is the main disease we are concerned about," Wise says. "However, if you see a little bit of gray leaf spot in the field when scouting, that doesn't necessarily mean you need a fungicide application."

Growers should look through the entire field, consider the type of field and where the lesions are located on the plant, she says.

"Check to see if lesions are on the ear leaf or if the disease is on a few leaves below the ear leaf and determine the percentage of plants affected," she says. "Fungicide application should be considered on fields where the hybrid is rated as susceptible or moderately susceptible, and 50% or more of the plants display disease lesions on the third leaf below the ear and higher prior to tasseling."

So far, Wise says she has seen gray leaf spot on lower leaves in some susceptible hybrids.

When looking at the previous crop and cropping system, it's important to remember that most fungal diseases, such as gray leaf spot, survive from year to year on crop residue, Wise said.

"Planting continuous corn or planting corn into high levels of corn residue, with no-till, will increase the likelihood for a disease to develop," she says.

Late planting across much of Indiana, Illinois and Ohio this spring means the crop is at increased risk of gray leaf spot development, according to research from Iowa State University, Wise says.

Favorable weather conditions for disease development of gray leaf spot are high humidity and moisture levels, as well as moderate to warm temperatures.

Wise cautions growers to be careful in making the decision about whether to apply a fungicide and also include economics in the list of things to consider.

"It's an added cost and with the price of corn right now, we need to make sure that the fungicide application will pay for itself," she says.

Economic return depends on fungicide application costs, drying costs and the price of corn. Wise says an increase of approximately 10 bu./acre is needed to pay for the cost of fungicide application with corn priced at $3/bu. and an application cost of about $30/acre. As the price of corn increases, the additional amount of yield needed to pay for the fungicide application decreases, Wise explains.

So when corn is at $3.50/bu. and application costs $30/acre, a yield increase of about 8.5 bu./acre is needed to pay for the cost of application. If corn reaches $4/bu., then a yield increase of about 7.5 bu./acre is needed to cover application costs.

For more information about determining whether a fungicide application is warranted, contact Wise at 765-496-2107 or

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