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Flushing Out the Fungicide Triangle

For a moment, flash back to the most basic concept of Plant Pathology 101: the disease triangle.
Host + pathogen + environment = disease. For a plant disease to occur, all three factors must first converge. For example, gray leaf spot infestations entail the crop (corn), pathogen (fungal spores), and environment (wet and warm weather) occurring simultaneously. If one is absent, there’s no disease. 

Kiersten Wise, Purdue University Extension plant pathologist, notes that another triangle has spurred corn and soybean fungicide use over the past decade. 

Components include: 

  • Changing cropping practices. The abundant residue produced by no-till and reduced-till systems creates a disease-pathogen paradise. 
  • Increased crop value. Were you fretting about $4-per-bushel corn this winter? Ten years ago, you’d have been popping champagne corks. It wasn’t that long ago since $2 corn and $6 soybeans were the norm. Fungicides have helped protect this higher value. 
  • New fungicide products. Several companies have developed new strobilurin and triazole fungicides and fungicide mixes in the past decade. 

The sum of these? Two hundred million cumulative corn and soybean acres have received a fungicide treatment over the past 10 years. Before then, fungicide use in corn and soybeans was sporadic, at best. 

The rub 

No one doubts that corn and soybean fungicides help you manage fungal diseases. 

“There are a lot of positives with fungicides,” says Daren Mueller, Iowa State University Extension plant pathologist. He says fungicides help protect yield that otherwise would be lost to fungal disease. 

Disagreement exists, though, over physiological perks offered by the strobilurin group of fungicides (including Headline, Quadris, Evito, Aproach) and strobilurin premixes (including Quilt Xcel, Stratego, Avaris, Aproach Prima, Priaxor, and Headline Amp). 

Back in the 1990s, European chemical company scientists observed that cereals treated with strobilurin fungicides were greener than untreated ones. By slicing ethylene production, these fungicides enabled plants to stay greener longer. Ultimately, this spurred more
photosynthesis and, ultimately, higher yields. Digging deeper, other strobilurin fungicide perks surfaced that include: 

  • More uniform soybean and corn seed size. 
  • Improved cornstalk strength. 
  • Better soybean seed quality. 
  • Better corn harvestability. 
  • Improved cornstalk disease tolerance. 

In 2009, BASF obtained a federal supplemental label stating these benefits under its Plant Health banner. 

Syngenta also promotes physiological benefits for strobilurin fungicides. “When it’s dry, there is not much disease, but plants still get some physiological benefits,” says Eric Tedford, Syngenta technical product lead for fungicides. “Quilt Xcel (a strobilurin and triazole mix) improves the water-use efficiency so that plants can better use the water they have.” 

Strobilurin fungicides help plants make better use of resources, adds Randy Myers, fungicide product manager for Bayer CropScience. “Farmers are paying a lot for seed, diesel fuel, and rent to put a crop in the ground,” he says. “Fungicides help protect that investment.” 

These perks are real, says Paul Vincelli, University of Kentucky Extension plant pathologist. It’s just the consistency of these benefits that gives third-party plant pathologists and agronomists pause. 

In university tests, strobilurin fungicide responses in the absence of corn disease have soared above 25 bushels per acre at times. 

“This is in line with what some farmers have told us,” he says.
In other cases, though, no yield advantage occurs. “We have worked hard to identify patterns where growers can make a decision,” says Vincelli. “Only a few patterns surface. There still are nagging uncertainties about when and where cost-effective physiological benefits will occur.” 

Applying a fungicide in the absence of disease also raises the specter of fungicide resistance. This has already happened in soybeans with frogeye leaf spot, a fungal disease that’s resisted strobilurin fungicides in 10 states. So far, no fungi that resist strobilurin fungicides have been confirmed in corn. 

So what should you do? To help you make the decision that’s best for your operation, turn this page and open the flaps. Inside the May issue of Successful Farming magazine, you’ll find answers to 25 questions you may have about fungicide use in corn and soybeans. For even more information, visit

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