An Overview of White Mold

Updated in 2026, this version replaces the 2020 An Overview of White Mold publication.

White mold (also called Sclerotinia stem rot) is a significant soybean disease in the northern United States and Canada (Figure 1). Caused by the fungus Sclerotinia sclerotiorum, white mold incidence and severity tend to be greater in years and regions where weather conditions are cool and wet during the growing season. White mold can significantly reduce yield, especially when climate and management practices favor high yield potential.

Developing an integrated white mold management strategy informed by field history and the most effective disease management practices can help reduce losses from this disease. While it is difficult to completely control white mold, integrating several management practices, including cultural control, varietal resistance, and chemical and biological control, can be part of an effective white mold management plan.

Signs and Symptoms

Sclerotinia sclerotiorum produces fungal structures called apothecia that form on the soil surface prior to symptom expression in the field. These apothecia are produced from sclerotia, which are hard, black survival structures that resemble mouse droppings residing in the soil. Apothecia can be easily confused with other non-pathogenic fungi, such as the common bird’s nest fungus (Figure 2).

Symptoms of white mold include water-soaked stem lesions that progress above and below infected nodes and eventually encircle the stem. Over time, infected stems are often killed and become bleached and stringy (Figure 3). Lesions can also occur on pods, petioles, and, in rare cases, on leaves.

Severe infection weakens the plant and can result in wilting, lodging, and plant death (Figure 4). White mold often occurs in patches in the field. Signs of the fungus that can assist in diagnosis include white cottony mycelia (fungal growth) and sclerotia (Figure 5) on and inside infected plant stems and pods. These signs of S. sclerotiorum and symptoms of white mold distinguish it from most other soybean diseases.

Development and Disease Cycle

The fungus that causes white mold, S. sclerotiorum, survives in the soil as sclerotia. When soil is shaded, moist, and cool, sclerotia within the top two inches (5 cm) of the soil profile can germinate and produce apothecia. Development of apothecia is favored when the 30-day average of maximum ambient air temperatures are below 68ºF (20ºC). An evaporative cooling effect in irrigated fields may allow apothecia development in warmer extended conditions (30-day average) of maximum air temperature > 80 ºF (27 ºC). Apothecia are small (approximately 1⁄8- to 1⁄4-inch (3-6 mm) in diameter), tan, cup-shaped mushrooms (Figure 2). Apothecia produce hundreds of spores called ascospores, which typically infect soybean plants via senescing flowers. Infection by ascospores is favored by maximum daily temperatures below 85 ºF (32 ºC) and frequent moisture from rain, fog, dew, or high relative humidity. A dense and nearly closed soybean canopy during flowering (growth stages R1 through R3) provides a favorable microenvironment for the development of white mold.

For white mold to develop, the field must contain the white mold fungus, the environment must be favorable for apothecial development, and a susceptible soybean variety must be present (Figure 6). Early canopy closure creates conditions favorable for apothecia production and plant infection. Factors that promote early canopy closure and favor white mold in soybean include narrow row spacing, high plant populations, and a high yield potential crop with a dense canopy. A field history of white mold and susceptible crops (most broadleaf crops) in the rotation greatly increases risk potential, as most disease inoculum originates within the field.

Yield Loss and Seed Infection

White mold causes yield loss by reducing seed number and weight. In addition, white mold can affect grain quality. Sclerotia may contaminate harvested grain (Figure 7, Left), which can result in price discounts for foreign material delivered at the elevator. Sclerotinia sclerotiorum can also infect soybean seed (Figure 7, Right) and serve as an important source of inoculum if planted into fields with no history of white mold. Infected seed can have reduced germination, and, in some cases, its oil and protein content can also be reduced.

Management

Incorporation of multiple management strategies is the best way to manage white mold.

Recordkeeping

Taking notes about where and how much white mold occurs in each soybean field is important for future disease management planning. Sclerotia can survive for at least eight years in soil. Tracking disease levels across years will also help determine the potential sclerotia inoculum density that may be present in a particular field. Recording disease and yield performance across different varieties will help in variety selection for fields with a history of white mold. Farmers with precision planting capabilities may also find it useful to adjust management practices based on previous yield maps in fields where white mold has occurred. This enables targeted fungicide applications and reduced planting populations for these specific areas.

Cultural Control

Crop Rotation

A minimum of two to three years of a non-host crop, such as corn, flax, or small grains (e.g., wheat, barley, or oat), can reduce the number of sclerotia in the soil. Forage legumes, such as alfalfa and clovers, are less susceptible to infection but are hosts for S. sclerotiorum. Soybean fields with a history of white mold should not be in rotations with broadleaf hosts such as edible beans, canola, cole crops (cabbage, broccoli, etc.), pulse crops (peas, chickpeas, and lentils), sunflowers, or potatoes.

Tillage

The effect of tillage on white mold development has been shown to be inconsistent. Deep tillage may initially reduce white mold incidence by distributing sclerotia deeper into the soil profile. However, sclerotia can remain viable for more than three years if buried 8-10 inches (20-25 cm) and may be returned to the soil surface in subsequent tillage operations. Although more sclerotia are found near the soil surface in no-till production systems, sclerotia may degrade faster in no-till systems.

Plant Populations

High plant populations contribute to dense, closed canopies and a greater risk of white mold. Use the lowest possible seeding rate that will achieve the recommended final plant density for your area. Local extension agronomists can help determine this number.

Row Spacing

Soybean planted into narrow rows may lead to faster and more complete canopy closure around the time of soybean flowering. Moving from 15-inch to 30-inch row spacing can sometimes reduce white mold severity, perhaps by as much as 50%. However, moving to wider row spacing can, in some cases, reduce yield potential compared to narrow row spacings.

Planting Date and Relative Maturity

Early planting, late-maturing varieties, and varieties with a bushy architecture or with a tendency to lodge can contribute to more closed canopies. However, the direct impact of these factors on white mold incidence and yield varies because disease development is highly dependent on environmental conditions during the reproductive growth stages.

Fertility and Plant Nutrition

High soil fertility, especially the use of nitrogen-rich manures and fertilizers, favors white mold development by promoting lush plant growth, early canopy closure, and lodging. The application of manure should be avoided on fields with a history of white mold.

Weed Control

Many common broadleaf weeds are hosts of S. sclerotiorum. High weed populations may also increase plant canopy density, favoring disease development.

Cover Crops

The use of small grain cover crops, such as oat, wheat, or barley grown with soybean can stimulate earlier emergence of apothecia compared to soybean grown alone. This can potentially reduce white mold incidence. Consider first how cover crops may affect soil moisture, nutrient availability, and shading before implementing them. In organic systems, planting into roller-crimped cereal rye cover crops can substantially reduce white mold incidence and severity. The mat of cereal rye left after roller-crimping produces a dark environment at the soil surface that is not conducive to complete apothecial development. The thick cereal rye mat may also function as a physical barrier limiting ascospore release and movement.

Irrigation Managment

Avoid excessive and frequent irrigation prior to and during flowering. Low moisture levels within the soybean canopy are critical for reducing the potential for white mold development. Infrequent, heavy watering is better than frequent, light watering.

Variety Selection

Moderately resistant soybean varieties are available. Although resistant varieties reduce disease severity, some level of white mold can still develop when conditions favor white mold. Plant the least susceptible variety in fields with a history of white mold.

Chemical Control

Fungicides (and some PPO inhibiting herbicides that contain the active ingredient lactofen) can be a part of an integrated management system for white mold. While some foliar-applied fungicides and lactofen have efficacy against white mold, none provide complete control.

Fungicides inhibit infection and growth of S. sclerotiorum, but how inhibition occurs depends on the product used. There are numerous products on the market that are labeled for white mold management. Table 1 includes products and programs that are most commonly used and have been evaluated across the North Central U.S. and Ontario, Canada. All effective products have limited upward movement in plant parts, and none move downward in the plant, where infection often occurs.

Table 1. Pesticide programs evaluated including pesticide active ingredient(s), group, and abbreviation. Source https://doi.org/10.31274/cpn-20191022-000 (Renfroe-Becton et al. 2026)

a Active ingredients within the same formula are separated by + while active ingredients of separate products are separated by a semi-colon.

b Total number of yield effect sizes generated from the primary analysis of variance, and used to evaluate each treatment in network meta-analysis.

c Total number of studies (environment) used to evaluate each treatment.

d fb = followed by

Herbicides containing lactofen as the active ingredient (e.g., Cobra or Phoenix) do not directly inhibit S. sclerotiorum, but may reduce white mold incidence. Lactofen can modify the soybean canopy and delay or reduce flowering, which may reduce infection by S. sclerotiorum. Lactofen can also induce a systemic acquired resistance (SAR) response that increases production of antimicrobial chemicals known as phytoalexins by the soybean plant that can inhibit the growth of S. sclerotiorum. Although these herbicides have potential benefits, their use  may also result in crop injury that can reduce yield on some soybean varieties. This happens particularly under hot and dry conditions that reduce soybean’s ability to recover from the injury and that are in years not conducive to disease development.

The Crop Protection Network (CPN) publication Pesticide Impact on White Mold (Sclerotinia Stem Rot) and Soybean Yield (CPN-5001) details the efficacy and economics of using modern pesticide programs. An interactive return on investment (ROI) tool has also been developed for pesticide programs for white mold. This tool is updated regularly and should be consulted periodically to understand which programs offer the best balance of efficacy with cost. Note that some of the more efficacious programs are often more expensive. Thus, the economics of using a particular program should be considered relative to your soybean yield potential, the risk for white mold damage, and grain sale price.

For more information about fungicides available for white mold management, consult the CPN publication Fungicide Efficacy for Control of Soybean Foliar Diseases (CPN-1019).

Application Timing

Fungicide must be applied at the proper growth stage to maximize efficacy for white mold control. Fungicide applications should be targeted from the R1 growth stage (beginning bloom) through to the R3 growth stage (beginning pod). The efficacy of fungicides for white mold management declines greatly after symptom development and/or the R4 soybean growth stage. An online resource has been developed to assist farmers in making the decision to apply fungicides at the optimum time. The resource is called the Crop Disease Forecasting web tool, which uses weather data and field location to provide an estimation of the risk of infection during the bloom period. These estimates of probability can help users decide whether to spray or not to spray.

Spray Coverage

Adequate plant coverage by pesticides deep in the soybean canopy where infections start is important for managing white mold with foliar fungicides. Flat-fan spray nozzles that produce fine to medium droplets (approximately 200-400 microns) provide the best fungicide coverage on sprayed plants. Follow manufacturers’ recommendations for spray volume and be aware of environmental conditions (such as wind speed) that influence coverage. Increase spray volume to improve coverage in fields with a thick canopy. Spray drones may also serve as a useful tool to push fungicides down into the soybean canopy, especially when the canopy is dense (e.g., R3 soybean growth stage).

Control Expectations

Chemical management strategies do not result in complete control of white mold and, therefore, should be considered only as one component of an integrated management program. Reduction of white mold incidence achieved by fungicides in university field trials has ranged from zero to approximately 60%.

Biological Control

Biological control can also be part of an integrated white mold management program. The fungus Coniothyrium minitans is the most widely available and tested biological control fungus and is commercially available as Contans. Application of C. minitans should occur at least three months before white mold is likely to develop, allowing adequate time for the fungus to colonize and degrade sclerotia (Figure 8). Degraded sclerotia will not produce apothecia and, therefore, will not produce ascospores to initiate infection of soybean. C. minitans should be incorporated as thoroughly as possible to a depth of two inches (5 cm). Avoid additional tillage that can bring non-colonized sclerotia to the soil surface.

There is limited data available on the efficacy of C. minitans for white mold management in soybean. In a few studies, the sclerotia number was reduced by as much as 95%, and the subsequent white mold incidence was reduced by 10-70%.

Biological control products such as Contans will not eliminate all sclerotia; fields heavily infested with sclerotia may continue to have disease development until the number of sclerotia in the soil is further reduced. More studies are needed to evaluate the efficacy of biological control products and their potential to reduce white mold of soybean, especially in fields with native populations of biological control fungi.

The core recommendations for managing white mold are:

Maintain records of field history and disease incidence of white mold.

Select soybean varieties carefully:

• Use varieties with the best available levels of resistance.

• Select the most appropriate maturity group for your region.

• Use pathogen-free seed.

Follow good cultural practices:

• Reduce plant populations and increase row width.

• Rotate with non-host crops (especially small grains)

• Consider reduced or no-till practices.

• Use cover crops to reduce inoculum density.

Fungicides may be warranted in fields with a history of white mold and where the risk of white mold is high. For best results, fungicide applications should occur between R1 and R3, before disease develops.

Consider biological control, which may be valuable as part of a long-term integrated management strategy to reduce sclerotia levels in a field.

Where irrigation is used, reduce frequency during flowering. Ensure irrigation is applied according to soil moisture requirements (i.e., avoid excessive irrigation events).

Acknowledgements

Authors

Damon Smith, University of Wisconsin-Madison; Carl Bradley, University of Kentucky; Martin Chilvers, Michigan State University; Travis Faske, University of Arkansas; Dean Malvick, University of Minnesota; Dylan Mangel, University of Nebraska; Daren Mueller, Iowa State University; Adam Sisson, Iowa State University; Richard Wade Webster, North Dakota State University; Kiersten Wise, University of Kentucky; Albert Tenuta, Ontario Ministry of Agriculture, Food and Agribusiness

Reviewers

Tom Allen, Mississippi State University; Alyssa Betts, University of Delaware; Mandy Bish, University of Missouri; Maira Duffeck, Oklahoma State University; Horacio Lopez-Nicora, The Ohio State University; Madalyn Shires, South Dakota State University; Darcy Telenko, Purdue University

Sponsors

The authors gratefully acknowledge funding for this project from the United Soybean Board; the North Central Soybean Research Program; USDA-NIFA CPPM, 2018-70006-28921; the Wisconsin Soybean Marketing Board; and the Michigan Soybean Promotion Committee. The authors thank the USDA and Grain Farmers of Ontario for their support.

How to cite: Smith, D., Bradley, C., Chilvers, M., Faske, T., Malvick, D., Mangel, D., Mueller, D., Sisson, A., Webster, R. W., Wise, K., Tenuta, A. 2026. An Overview of White Mold. Crop Protection Network. CPN-1005. doi.org/10.31274/cpn-20190620-030.