Frogeye leaf spot, which is caused by the fungus Cercospora sojina, is an important pathogen of soybean plants.

University of Illinois plant pathologist Carl Bradley said that since 2010, strains of the fungus that are resistant to strobilurin fungicide have been found in Illinois and other states.

“In 2012, we detected strobilurin fungicide-resistant strains of Cercospora sojina in several new states, counties, and parishes,” he said. “In total, we have detected strobilurin fungicide-resistant strains of Cercospora sojina in 44 counties or parishes in eight states since 2010.”

This year, Bradley’s research team conducted a soybean foliar fungicide trial at the U of I Dixon Springs Agricultural Center in a field with a history of frogeye leaf spot and a strobilurin fungicide-resistant strain of Cercospora sojina. They applied several different foliar fungicides from different chemistry classes at the R3 soybean developmental stage (beginning pod formation) and evaluated their efficacy in managing frogeye leaf spot.

They found that strobilurin fungicides were not effective, but fungicides from other chemistry classes reduced frogeye leaf spot severity.

What does this mean?

“As we plan for the 2013 growing season, it is important to consider the increasing number of detections of strobilurin fungicide-resistant strains of Cercospora sojina across several soybean-producing states and how this problem might affect frogeye leaf spot management decisions,” Bradley said.

He recommended several management practices to slow down the development of fungicide resistance, which include:

  1. Use resistant varieties and cultural practices (crop rotation, tillage) to help manage frogeye leaf spot. If disease levels are kept low using alternative management practices, fungicides may not be needed.
  2. Do not rely on only one class of fungicides to control plant diseases. Fungicides from different chemistry classes can be mixed together to reduce the selection pressure on the fungal population.
  3. Apply foliar fungicides only to control plant diseases; do not apply them for other reasons. Every time a fungicide is applied, it exerts a selection pressure on the fungal population, and individual fungal isolates may be selected that are not as sensitive to the fungicide. Keeping this selection pressure to a minimum is important in prolonging the effectiveness and lifespan of a fungicide.