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Corn+Soybean Digest

Questions Remain About Fungicides on Corn

Fungicide use on corn has increased in Indiana over the last few years, says Kiersten Wise, Purdue University Extension plant pathologist.

This increase is, in part, because fungicides help control disease, and fungicides containing a strobilurin mode of action might have physiological effects on the plant that are unrelated to disease control and sometimes increase yield, Wise says.

"We've seen a shift in corn management where growers are applying these fungicides in hopes that they'll get higher yields as a result," Wise says. "That's one of the factors that has driven the increase in applications."

However, Wise was quick to add that there is little evidence at present as to whether those physiological effects consistently increase yield.

"We know that fungicides are good at controlling foliar diseases, but we struggle to see a consistent yield increase from a fungicide application in the absence of disease," she says.

When managing disease problems, there are five main factors that should be taken into consideration for spraying a fungicide. Those factors include: level of disease in the field, hybrid susceptibility, previous crop and cropping system, weather conditions and late planting.

The level of disease in the field is one of the most crucial factors, and scouting fields prior to tassel emergence can help determine the level of disease pressure in a field, Wise says.

"This factor can make the difference in whether or not a fungicide application will pay at the end of the season," she says.

Hybrids vary in their susceptibility to foliar diseases of corn, and hybrids susceptible to diseases are at a greater risk of disease development than hybrids with moderate to high levels of disease resistance, Wise says.

"Most of the fungal diseases, such as gray leaf spot, survive year to year on crop residue, and planting corn-on-corn or planting corn into high levels of corn residue will increase the likelihood that disease will develop," she says.

Gray leaf spot is a perennial and economically damaging disease in the U.S. caused by the fungus Cercospora zeae-maydis. It is now one of the most important foliar diseases in Indiana. Other common diseases include anthracnose leaf blight and common rust.

Those diseases require high humidity, moisture and moderate to warm temperatures for disease development, Wise says.

It is important to check the plants for these diseases to control any potential problems. Early symptoms of gray leaf spot are small, pinpoint lesions surrounded by a yellow halo.

"We recommend scouting before tasseling to determine what diseases are present," she says. "If the disease is at a level where a fungicide application would be warranted, a fungicide application may be profitable."

The best time to apply a fungicide typically is at tasseling or shortly after. Research has shown that late applications of fungicide may not be profitable. Fungicide application costs vary but, for use in corn, $20-30/acre is average.

If farmers have questions about fungicide use, they should contact Wise at 765-496-2170,

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