White mold, also known as Sclerotinia Stem Rot, is a widespread disease that causes substantial reductions in yield and negatively impacts seed quality in soybeans globally. It is a major concern for farmers, particularly in the northern United States and Canada. The disease’s severity fluctuates annually, as its development is highly dependent on specific environmental conditions and management practices. White mold is distinguished by fluffy, white fungal growth on infected plant tissue and hard, black survival structures, which are key to its life cycle.
The Causative Pathogen
White mold is caused by the fungus Sclerotinia sclerotiorum, a pathogen with a broad host range that infects over 400 plant species. This fungus spends the vast majority of its life cycle in the soil as hardened, melanized resting bodies called sclerotia. Sclerotia are dark, irregularly shaped structures that act as the primary survival mechanism for the pathogen.
These structures are resistant to harsh environmental conditions, including prolonged freezing, and can remain viable in the soil for extended periods, often for more than three years. The melanized rind protects the internal fungal tissue from degradation. When sclerotia are near the soil surface, they wait for the precise combination of temperature and moisture to initiate the next phase of the disease cycle.
Environmental Conditions Required for Infection
The activation of sclerotia and subsequent infection require a precise set of environmental conditions, most notably cool temperatures and high moisture. Sclerotia near the soil surface must first undergo carpogenic germination, favored by prolonged periods of cool, moist soil conditions. This process yields small, tan, mushroom-like structures called apothecia.
The apothecia discharge millions of microscopic, airborne spores known as ascospores into the crop canopy. These ascospores are the primary source of infection for soybeans, but they cannot directly infect healthy plant tissue. Infection occurs when ascospores land on dying flower petals, which provide the necessary nutrients for the spores to germinate and establish an infection. The soybean is most vulnerable during the reproductive growth stages, specifically R1 (beginning bloom) through R3 (beginning pod).
A dense crop canopy, often resulting from narrow row spacing or high plant populations, is a major contributing factor. A closed canopy creates a cool, humid microclimate that traps moisture and keeps the plant surface wet for the 12 to 16 hours required for the fungus to establish itself. Temperatures between 50°F and 70°F (10°C and 21°C) during flowering, combined with frequent rain or extended dew, increase the risk of an outbreak.
Identifying White Mold in the Field
The first noticeable symptoms often appear several weeks after initial infection, typically around the R5 or R6 growth stages, and frequently in patches. Initial signs are water-soaked lesions that develop on the main stem at the nodes, where old flower petals have dropped. These lesions quickly expand, girdling the stem and causing the plant to wilt, with the leaves turning gray-green and eventually brown.
The distinguishing feature is the appearance of fluffy, white, cottony fungal growth, known as mycelium, on the surface of the infected stems and pods. As the disease progresses, the stem tissue becomes bleached and may appear shredded. Hard, black sclerotia form within this fungal mass, often inside the hollowed-out stem. These new sclerotia fall to the soil at harvest, completing the disease cycle and serving as the inoculum for future seasons.
Management and Control Strategies
Managing white mold requires an integrated approach focusing on cultural practices and chemical intervention, as no single strategy offers complete control. Since a dense canopy promotes the disease, cultural controls aim to increase airflow and reduce humidity. Increasing row spacing and reducing planting density helps delay canopy closure and reduces the favorable microclimate.
Crop rotation must involve non-host crops like corn or wheat, not susceptible broadleaf crops such as canola or edible beans. Since sclerotia survive for years, a single year of rotation may not eliminate the inoculum. Selecting soybean varieties with genetic tolerance is one of the most effective preventative measures, especially in fields with a known disease history.
Fungicide application is a common chemical control measure, but timing is critical for efficacy. Fungicides are not curative and must be applied preventatively to protect the flowers from airborne ascospores. The optimal window for application is during the R1 to R2 growth stages (beginning to full bloom), when the soybean plant is most susceptible. Applying fungicide after the R3 stage provides little benefit.
Fields with a history of white mold should be managed to prevent the spread of sclerotia by harvesting affected areas last to minimize movement on equipment. Tillage is complex; deep tillage can temporarily bury sclerotia, but it may also bring viable sclerotia back to the surface in subsequent years. A proactive strategy combining varietal selection, cultural adjustments, and timely fungicide application offers the best protection.