Key Takeaways
Flowering is the critical window. Soybeans are most susceptible to white mold during flowering, making this the time to assess risk and consider management options.
Warm weather has delayed disease development, but conditions can change. Continue scouting fields with a history of white mold, dense canopies or irrigation, especially after rainfall and extended canopy moisture.
Timing matters. Fungicides are most effective when applied during the flowering window before infection occurs; late applications will not protect infected plants.
Learn more: Watch White Mold Update – Before You Spray for timely white mold scouting and management recommendations.
As soybean across Nebraska begins flowering, now is the time to evaluate white mold risk and determine whether fungicide applications may be needed. Although the 2026 growing season has been warmer than average and no apothecia — the small mushroom-like structures that release white mold spores — have been observed during field scouting to date, rainfall and extended periods of canopy moisture can quickly increase disease risk.
Most soybean fields are currently at the R1 (beginning bloom) growth stage, with canopy closures ranging from approximately 50%-85%. While the recent hot weather has likely delayed apothecia development, growers should continue monitoring fields, especially those with a history of white mold, dense canopies, irrigated production, or cooler and wetter conditions. White mold significantly impacted yields in 2021 and 2023, and proactive management is crucial for this season.
The key factors to keep in mind are:
- Flowering Susceptibility: Soybean plants are most vulnerable to white mold during the flowering stage.
- Protective Measures: Protect flowers during the flowering period. Once infection occurs, fungicides cannot save infected plants.
- Environmental Conditions: Wet, humid environments facilitate the spread of white mold.
- Field History: Fields with a prior history of white mold are at greater risk of occurrence, often beginning in the same hotspots where infection occurred in past seasons due to the persistence of sclerotia in the soil.
For detailed management strategies, including genetic resistance, cultural practices, and chemical/biological control, please refer to the sections below.
Pathogen Overview
Sclerotinia stem rot, commonly known as white mold, is a fungal disease caused by Sclerotinia sclerotiorum. The fungus survives in soil over winter as hard, dark structures called sclerotia. In spring, saturated soils and a dense canopy favor the development of apothecia (Figure 1), which release millions of airborne spores during flowering.
These spores infect soybean primarily through aging blossoms and injured tissue, requiring free moisture for infection. The disease often prevents pods from developing and produces more sclerotia on and in the stems, which are scattered back onto the soil during harvest and can survive for several years.
Disease Symptoms
Symptoms appear during pod development, starting with wilting and discoloration of leaves. Infected stems and pods show white fungal growth (mycelium) and sclerotia. Water-soaked lesions typically begin near infected flowers before spreading through the stem. Stems become tan, bleached, pithy and shred easily (Figure 2). Symptoms typically occur 6 to 14 inches above the soil but can also affect the plant base.
Favorable Environmental Conditions
White mold develops from flowering to pod formation under wet, humid conditions with moderate temperatures (less than 85°F). It is more prevalent in northern counties and irrigated fields with consistent moisture and cooler canopy temperatures.
Management Strategies
Genetic Resistance
Choose varieties rated against white mold, avoiding highly susceptible ones, and select shorter varieties that do not lodge.
Cultural Practices
In fields with a history of significant white mold, use wider row spacing (30-inch over 15-inch) to delay canopy closure and reduce humidity. Avoid irrigation during flowering if possible. Rotate with non-host crops like corn and wheat, and maintain good weed control as some weeds and cover crops can host the fungus. Although tillage has been considered for white mold management, it is generally not recommended because it provides limited control and can increase the risk of soil erosion and moisture loss.
Manganese management is another consideration, as improper soil manganese levels may weaken a plant’s defense against white mold. Soil manganese should be adjusted before the growing season when needed. Foliar manganese applications can be made during the season, although research on their effectiveness is limited. Any foliar feeding should be done before flowering begins.
Chemical/Biological Control
Apply foliar fungicides during flowering when fields have a history of white mold and weather conditions favor disease development. Applications made after the infection window are unlikely to provide effective control. Refer to the Crop Protection Network and the Nebraska Extension NebGuide EC130, “Guide for Weed, Disease, and Insect Management in Nebraska” for fungicide efficacy test results to find products that are effective on the pathogen.
