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Soybean frogeye leaf spot resistance to strobilurin fungicides is increasing

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
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. Don't 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.

For more information, read The Bulletin


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