The 2025 soybean growing season is facing an unexpected challenge: an early and aggressive outbreak of major foliar and stem diseases across the Midwest. Conditions driven by heat, humidity, and dense canopies have accelerated infections by frogeye leaf spot, Cercospora leaf blight, sudden death syndrome (SDS), and red crown rot (RCR)-some appearing further north than usual.

"This is not a typical July," warned Daren Mueller, soybean plant pathologist at Iowa State University. "Frequent rains and high humidity have accelerated diseases that usually emerge in August."

The Disease Triangle: More Than Just Rain
Experts like Stephanie Porter of the Illinois Soybean Association stress that moisture doesn't have to come from rain. "High humidity, dew, and irrigation can be enough," Porter explained. "In dense canopies, those leaf surfaces stay wet for hours, creating ideal conditions for infection." The "disease triangle"-a susceptible host, a virulent pathogen, and a conducive environment-is firmly in place.

Porter advises growers to scout frequently and not rely solely on rainfall data or weather forecasts. "The canopy microclimate often tells a different story than your weather app," she said.

Frogeye and Cercospora Advance North
Albre Brown from BASF reports that frogeye leaf spot, once considered a Southern disease, is now active in central Illinois and Indiana. Combined with Cercospora, these diseases threaten both yield and seed quality. In 2024, Cercospora was linked to 3.2 million bushels of U.S. soybean losses, per the Crop Protection Network.

"Frogeye is aggressive," Brown noted. "It spreads rapidly in these conditions, especially in later-planted fields maturing under peak heat and humidity."

Red Crown Rot: The Hidden Menace
Often misidentified as SDS, red crown rot is reappearing across Illinois, Indiana, and Kentucky. Identified by tiny red perithecia at the stem base, RCR can persist in soil for years and infect multiple hosts, making it resilient to standard crop rotations.

Porter emphasized the importance of accurate diagnosis: "You can't fight what you don't correctly identify. If red crown rot is in your field, rotating out of soybeans may not be enough."

Fungicide ROI and Best Practices
As growers weigh their fungicide options, Brown reported that even at $10 per bushel, fungicides offer positive returns. Trials showed average yield gains of 5 bushels per acre, with some areas reaching 7 bpa under heavy pressure.

Timing is crucial:

• Cercospora: Target late R2 for full reproductive window coverage.

• Frogeye: Apply between R3 and R5 to protect yield-critical leaf tissue.

Rotating modes of action is vital to prevent resistance. Brown recommends using multisite fungicides with multiple FRAC groups. Many growers also tank-mix insecticides to address rising stink bug and Japanese beetle activity-but Porter urges verification before spraying: "Scout before you mix. Don't treat edge damage like a whole-field threat."

Tools like the Crop Disease Forecasting Tool and Fungicide Efficacy Tool from the Crop Protection Network can support informed decisions.

Midwest Growers: Eyes on the Canopy
Porter sums it up best: "Every field is different. If there was ever a year to be proactive, this is it. Get out there, scout, and make decisions based on what's really happening-not just what you expect."