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How to make a corn fungicide decision

Getting an itchy trigger finger for applying fungicides on your corn?

Consider fungal disease levels above all else, says Kiersten Wise, Purdue University Extension plant pathologist. Wise spoke at this week's Top Crop Farmer Workshop at Purdue University.

"Where there is significant gray leaf spot (GLS) present in a field, we see a good response from a fungicide application," says Wise.

Higher fungicide and application costs and this summer's sinking prices make this year's decision on corn tougher. Wise notes product and application costs are hovering around $30 per acre this year, up from the mid-$20s last year. Summertime cash corn prices are also sinking.

"With the price of corn this year, it's more of a risk," she says.

Greg Shaner, retired Purdue University Extension plant pathologist, compiled 68 university corn fungicide trial results from 13 states and one Canadian province in 2008. Breakeven calculations were derived based on product and application cost, and the frequency of trials in which yields exceeded breakeven thresholds.
At $4 per bushel corn and $28 per acre fungicide and application costs, the breakeven level is 7 bushels per acre. The frequency of reaching this breakeven level is 34.3%.

At lower corn prices, payback odds decrease. At $3 corn and the same application cost, your breakeven level increases to 9.3 bushels per acre. The frequency of reaching this breakeven level is 26.3%.

Your own situation could heighten or lessen the odds of a fungicide payoff, though. Risk factors that can lead to significant fungal disease like GLS include:

  • No-till and/or continuous corn. The increased residue can create a haven for fungal disease pathogens.
  • Disease-susceptible hybrids.
  • Late planting. This increases the risk of fungal diseases like GLS that cause yield loss, Wise says.
  • Favorable weather conditions. GLS thrives under high humidity and moisture levels and moderate to warm temperatures.

"When it is hot and dry, we haven't seen the yield response (from fungicides) because conditions are less favorable for disease development," she says.

Scouting your fields can also help you determine if an application is warranted. In Indiana, GLS is the main fungal disease that clips corn yields. Just because corn exhibits some GLS lesions doesn’t necessarily justify a fungicide application, Wise says.

She advises using scouting thresholds developed by Gary Munkvold, an Iowa State University plant pathologist. First, check 100 plants across the field before tasseling in the V10 to V14 stage. Check for lesions on the ear leaf or if the disease is on leaves below the ear.

Treatment decisions hinge on two factors:

  1. Fifty percent or more of the plants display GLS lesions on the third leaf below the ear or higher prior to tasseling.
  2. Hybrid susceptibility. "We often don't recommend treating hybrids with high disease resistance to gray leaf spot," Wise says.

Fungicides containing a strobilurin mode of action like Headline, Quadris, Quilt, and Stratego can have plant physiological benefits unrelated to disease control. However, these benefits by themselves don't justify a fungicide application, says Wise. Significant fungal disease must first occur.

"University data shows there is not a consistent benefit (to fungicide application) without disease present," says Wise. Although these fungicides give good disease control, their physiological effects do not always result in yield increases that pay for the cost of the fungicide application.

That's why fungicides should be applied for their primary purpose -- fungal disease control, says Wise.

Getting an itchy trigger finger for applying fungicides on your corn?

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