Considerations for fungicide use in soybean

July 8, 2020 11:21 AM
Blog Post

Now is the time when decisions to apply a foliar fungicide in soybeans are being made. As you are making that decision, this is a quick reminder that fungicide resistance, particularly to the quinone outside inhibitor (QoI) fungicides (FRAC Code 11), has been found to be widespread in Iowa in the pathogen that causes frogeye leaf spot on soybean. See this previous article for additional details.

What does this mean for selecting fungicides? This means that some fungicides may not work as well as others for managing frogeye leaf spot. A little bit of homework is required if you want to use the most effective fungicides. See the 2020 fungicide efficacy table on the Crop Protection Network for more information. Additional information on how fungicides are classified by their modes of action can be found here.

Foliar fungicides applied during late flowering and early pod set to pod filling stages can reduce frogeye leaf spot. However, to date there has been very little frogeye leaf spot or other foliar diseases in Iowa soybean fields. Hot, dry weather is not ideal for fungal growth. Continue to monitor weather conditions for the best time to spray.

If you believe fungicide resistance may be an issue in your field, contact your local extension field agronomist.

Symptoms of frogeye leaf spot include gray lesions with reddish-brown borders on soybean leaves. More information about frogeye leaf spot and other foliar diseases of soybean can be found at the


Daren Mueller Associate Professor

Daren Mueller is an associate professor and extension plant pathologist at Iowa State University. He is also the coordinator of the Iowa State Integrated Pest Management (IPM) program. Daren received his bachelor's degree from the University of Wisconsin-Madison in 1996, and his master's degree a...