Watch for Frogeye Leaf Spot in soybeans

It’s now common in Midwestern soybean fields.

Frogeye leaf spot used to be a Mid-South disease, ravaging soybean fields in states like Tennessee.

No more. This fungal disease now infests soybeans in Midwestern states like Iowa.

“Its severity will vary from year to year, but it is here to stay,” says Dean Grossnickle, a central Iowa agronomist for Syngenta. 

Frogeye leaf spot is bad enough. What’s worse news is that it increasingly resists Group 11 Qol (strobilurin) fungicides, a leading mode of action.

“We have to assume it (resistance) is widespread and manage appropriately,” he says. 

Disease triangle

Whether or not frogeye leafspot surfaces hinges on the disease triangle:

  • Host
  • Pathogen
  • Environment 

“States like Iowa have the host and pathogen, so all that’s needed is environment for it to blow up,” says Grossnickle.

Frogeye leaf spot thrives under heavy dew conditions and 80°F. to 86°F. temperatures. “If weather stays cool, infection rates decrease,” says Grossnickle.

Visible symptoms surface seven to 14 days after infection, says Grossnickle. Lesions normally surface on leaves as small, dark, water-soaked spots, note University of Minnesota (U of M) plant pathologists. They develop into brown spots surrounded by a darker reddish-brown or purple ring. Lesion centers turn light brown or light gray as they age. The center of spots may turn white with black specks visible (fungal fruiting structures) or the centers may fall away leaving a shot-hole appearance. The lesions may eventually merge, covering large areas of the leaves and resulting in defoliation.

“Left unchecked, yield losses have tallied up to 30% in Southern states when the disease is severe,” says Dean Malvick, a U of M Extension plant pathologist.

“It is more likely to infect new tissue on a plant vs. that on older leaves, so it will show up in the upper part of the canopy,” says Grossnickle. “It will continue to increase as long as weather is favorable.”

Even if symptoms appear, though, yield potential can still be salvaged with a triazole (Group 3) fungicide. However, relying on one mode of action is what caused frogeye leaf spot to resist strobilurin fungicides, says Grossnickle. That’s why including both a triazole and SDHI (Group 7) fungicide is recommended. Fungicide works best when applied at the R3 (beginning pod) stage, he says.

Grossnickle advises farmers to familiarize themselves with Fungicide Resistance Action Committee (FRAC) Group numbers to identify fungicide groups.  Group number examples include

  • Group 11 that include Qol (strobilurin) fungicides 
  • Group 7 that include SDHI fungicides. 
  • Group 3 that include triazole fungicides.

“If a fungicide has a Group 11 label, that fungicide will not be active on frogeye leaf spot,” says Grossnickle. 

Prevention tactics

Ways to reduce the incidence of frogeye leafspot include:

  • Burying infected residue via tillage.
  • Rotating to a nonhost crop like corn. 
  • Planting soybean varieties resistant to frogeye leafspot. 

The degree of frogeye resistance can differ between varieties, says Grossnickle. He recommends farmers work with a seed salesperson to find the varieties that best fit their farm.

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