White Mold in Soybeans: Scouting, Prevention, and Fungicides

White mold, caused by the fungus Sclerotinia sclerotiorum, is one of the most damaging diseases soybean growers face in cool, humid production regions. The fungus attacks plants through their flowers during bloom, eventually killing stems and pods while leaving behind hard, dark survival structures called sclerotia that can persist in soil for years. Yield losses in heavily infected fields can reach roughly 10 bushels per acre, and the disease is difficult to manage because no single tool eliminates it. Effective control depends on stacking several strategies together, from crop rotation and seeding rate to well-timed fungicide applications and, increasingly, predictive weather-based tools.

How the Fungus Survives and Attacks

The white mold cycle begins in the soil. S. sclerotiorum produces sclerotia, small dark structures roughly the size of a rat dropping, that serve as the fungus’s long-term survival vehicle. When conditions are right, sclerotia near the soil surface germinate and produce small, mushroom-like fruiting bodies called apothecia. Each apothecium releases hundreds of microscopic spores (ascospores) into the air, and these spores are the primary way the fungus reaches soybean plants. Ascospores typically land on and infect senescing flowers, which provide the nutrients the fungus needs to gain a foothold before colonizing stems and pods.

Two factors govern whether sclerotia survive long enough to cause trouble. Depth matters enormously: sclerotia sitting on the soil surface retain viability far longer than those buried even a few inches down. In a multi-year field study in New York, about 15% of surface sclerotia were still viable after more than two years, while buried sclerotia lost viability more quickly and none survived past roughly 670 days.¹ Work from India found an even steeper decline under natural field conditions: sclerotia buried below 5 cm for six months showed zero germination, while those left on the surface still germinated at over 80% after three months.² The practical takeaway from both studies is that at least two years of rotation away from susceptible crops is needed to meaningfully reduce the sclerotia bank in a field.

Why Cool, Humid Conditions Drive Outbreaks

White mold is not a warm-weather disease. Apothecia develop best when the 30-day average of daily maximum air temperatures stays below about 68°F (20°C). In irrigated fields, evaporative cooling under the canopy can allow apothecia to form even when ambient highs exceed 80°F. Once ascospores are airborne, infection is favored when daily maximum temperatures stay below about 85°F and the canopy stays consistently moist from rain, dew, fog, or high humidity.³

This is why canopy density matters so much. A thick, closed soybean canopy during bloom traps moisture and keeps temperatures lower near the soil surface, creating exactly the microclimate the fungus prefers. Fields planted at high seeding rates, with narrow rows, or with tall, bushy varieties tend to close their canopy earlier and hold humidity longer, setting the stage for severe outbreaks.

How Oxalic Acid Gives the Fungus Its Edge

Once ascospores land on a flower and begin to grow, the fungus relies on a chemical weapon called oxalic acid to overpower the plant’s defenses. Oxalic acid is the single most important virulence factor S. sclerotiorum possesses. Laboratory work showed that oxalate-deficient mutants of the fungus could still form initial lesions on plant tissue, but those lesions did not expand. Without oxalic acid, the fungus essentially stalls after first contact.⁴

The reason oxalic acid is so effective is that it disarms the plant’s natural immune response. Healthy plants detect invading fungi and mount an “oxidative burst,” flooding the infection site with reactive oxygen species to kill the pathogen. Oxalic acid suppresses that burst. Research in tobacco and soybean cell lines found that oxalate inhibited hydrogen peroxide production at concentrations lower than what the virulent fungus naturally produces.⁵ In a cruel twist, once oxalic acid has suppressed the initial defense, it then triggers programmed cell death in plant cells, effectively convincing the plant to kill its own tissue ahead of the advancing fungus.⁶ Understanding this mechanism explains why resistance breeding has been so difficult: the fungus hijacks the plant’s own cellular machinery rather than relying on a single point of entry that breeders can block.

Cultural Practices That Reduce Risk

Because no single management tool is a silver bullet for white mold, most integrated programs start with cultural practices aimed at reducing inoculum and making the field environment less hospitable to the fungus.

Crop rotation: Growing a non-host crop (corn, small grains, or grass forages) for at least two consecutive years helps deplete the sclerotia bank. The New York survival study confirms that buried sclerotia lose viability faster than surface ones, so combining rotation with tillage that buries sclerotia can accelerate the decline.
Seeding rate: Lower seeding rates delay or reduce canopy closure, allowing more air movement and sunlight penetration during bloom. Field trials found that plots with lower seeding rates generally had less white mold, though growers need to balance disease avoidance against the yield potential of thinner stands.⁷
Tillage: Moldboard plowing buries sclerotia deep enough to reduce their viability, but it also brings old buried sclerotia back up when the field is plowed again. No-till systems tend to leave sclerotia on the surface, where they remain viable longer, yet apothecial numbers have been observed to be lowest in no-till fields compared with moldboard-plowed ones.⁸ The relationship between tillage and white mold is not straightforward: what matters most is where the sclerotia end up relative to the surface in any given year.
Row spacing and variety selection: Wider rows and shorter, less bushy varieties reduce canopy density at bloom. Some growers in high-risk fields also avoid planting their earliest-maturing groups, which tend to flower during the coolest part of summer when conditions favor the disease.

Fungicide Timing and Profitability

Chemical control remains the most widely used in-season tool for managing white mold, but the economics of fungicide application are highly dependent on how much disease pressure actually materializes.⁹ The flowering window (roughly growth stages R1 through R3) is the critical spray window, because that is when senescing petals create the entry point for ascospores.

A 10-year meta-analysis of cooperative fungicide trials across the north-central United States found wide variation in performance among products. The least effective product tested reduced disease incidence by about 54%, while the most effective combination cut incidence by nearly 84%. Yield responses ranged from roughly 323 kg/ha (about 4.8 bushels per acre) for the weakest performer to about 626 kg/ha (roughly 9.3 bushels per acre) for the strongest.¹⁰ But the crucial detail is that disease pressure in the untreated check plots explained much of the variation. When untreated disease incidence exceeded 30%, the yield benefit of spraying jumped by an estimated 352 kg/ha compared with the low-disease scenario. In high-disease situations, the more effective (and more expensive) fungicides had a greater than 65% probability of paying for themselves. In low-disease years, profitability was far less certain and depended heavily on fungicide cost and soybean price.

A separate meta-analysis looking at disease severity index rather than incidence found that yield loss does not really kick in until about 20 to 25% severity, but at 68% severity, losses reach roughly 10 bushels per acre.¹¹ That threshold pattern matters for decision-making: spraying a field that is unlikely to exceed 20% severity is probably burning money, while waiting too long in a field that is trending toward 50% or above can cost hundreds of dollars per acre in lost yield.

More recent work evaluating specific programs found that a two-pass application of boscalid (Endura) at the R1 and R3 growth stages provided the best overall disease suppression, but a single application of lactofen (Cobra) at R1 offered the best balance of disease reduction and affordability for many growers.¹² That research has been built into an interactive online tool that lets growers plug in their own fungicide costs and expected soybean prices to compare the probability of different programs paying off.

Using Weather-Based Forecasting Tools

One of the biggest advances in white mold management in recent years is the development of weather-driven prediction models. The most widely known is Sporecaster, a decision support system that uses localized weather data to predict the risk of apothecia forming in a given field during the soybean flowering window.¹³ When the model indicates low risk, growers can skip the fungicide pass and save the expense. When it flags high risk, they can spray with more confidence that the investment will pay off.

Research is ongoing to fold soybean variety resistance ratings into these models. The idea is that a partially resistant variety planted in a field with a moderate weather-risk score might not need a spray, while a susceptible variety in the same field might. Integrating those layers could sharpen spray decisions further, though the models are still being refined and validated across different production regions.

Biological Control as a Complement

Biological control agents targeting S. sclerotiorum have been studied for decades, and a few have shown real promise in field settings. The most studied is Coniothyrium minitans, a fungus that parasitizes sclerotia in the soil. Its primary mode of action is mycoparasitism: it physically invades and degrades sclerotia before they can germinate and produce apothecia.¹⁴ In field trials, C. minitans reduced the white mold disease severity index by about 69% and cut the number of sclerotia in the soil by over 95%.¹⁵

Those numbers sound dramatic, but there are practical caveats. Biocontrol agents work on the sclerotia in the soil, not on ascospores that have already been released. They are best thought of as a long-term inoculum reduction strategy rather than a rescue treatment once you see white mold in the field. Application usually means incorporating the product into the soil well before planting, which adds a pass and a cost. Still, for fields with a chronic white mold history, building C. minitans into the rotation can meaningfully reduce the sclerotia reservoir over several seasons.

The State of Genetic Resistance

If you have ever wondered why there is no truly “resistant” soybean variety on the market for white mold, it is because resistance to this disease is partial, complex, and difficult to breed for. No single gene confers immunity. Instead, resistance is controlled by many small-effect genetic regions spread across multiple chromosomes. Genome-wide association studies have identified dozens of regions linked to resistance in different environments. One large study found 16 regions associated with field resistance and 11 with greenhouse resistance, though only one of those overlapped in both settings.¹⁶ That poor overlap between field and controlled environments is a persistent challenge: resistance that looks good in a greenhouse does not always hold up under real field conditions.

Validation work across multiple breeding populations has confirmed several resistance regions identified in earlier studies, with favorable genetic contributions from both cultivated varieties and plant introductions (wild or semi-domesticated relatives). In one set of populations, the broad-sense heritability of resistance ranged from 0.39 to 0.66, meaning that genetics explains somewhere between a third and two-thirds of the variation in disease response, with the rest driven by environment and management.¹⁷ Other mapping work identified three consistently detected resistance regions on chromosomes 6 and 20 that together explained 31 to 51% of lesion length variation, with the favorable versions coming from the variety Maple Donovan.¹⁸

Translating these findings into commercial varieties is slow work. Breeders need to stack favorable versions of many small-effect genes while also maintaining yield, maturity, and other agronomic traits. Progress is being made, and growers in white-mold-prone areas should pay attention to variety resistance ratings when they are available. But no variety is immune, and even partially resistant lines benefit from the cultural and chemical strategies described above.

Fungicide Resistance on the Horizon

Growers who rely heavily on fungicides should be aware that the tools they depend on are not guaranteed to work forever. S. sclerotiorum is currently sensitive to the main fungicide classes used against it, including succinate dehydrogenase inhibitors (SDHIs). But resistance to SDHIs has emerged in other fungal crop pathogens through point mutations in their target genes. Researchers have developed a system using S. sclerotiorum itself as a test organism to study how these mutations confer resistance, partly because the fungus is easy to grow and transform in the lab.¹⁹ The practical implication is that while resistance in white mold populations has not yet been documented as a widespread field problem, the genetic machinery for it exists, and over-reliance on a single fungicide class could accelerate its emergence. Rotating chemistries and combining fungicides with cultural practices is the best insurance against ending up with a field full of resistant sclerotia.

Scouting and Recognizing Symptoms

White mold can be easy to miss in early stages if you do not know what to look for. The first visible sign is usually a water-soaked, gray-green lesion on a stem at or near a node where a flower was attached. Within days, the lesion expands and white, cottony mycelium appears on the outside of the stem. As the disease progresses, the affected stem bleaches and dries out, and infected plants wilt from the lesion upward. Splitting an infected stem open reveals the sclerotia forming inside: hard, black structures that look almost like mouse droppings lodged in the pith. These sclerotia eventually fall to the ground during harvest and become the inoculum source for future seasons.

Symptoms often show up first in low spots, along field edges near tree lines where air movement is poor, and in areas where the canopy is thickest. By the time cottony mycelium is visible on the outside of stems, fungicide applications are too late for those particular plants. Scouting during bloom, before symptoms appear, is when decisions about spraying actually need to be made, which is why the weather-based forecasting tools are so valuable.

What Harvest Practices Spread (or Limit) the Problem

Harvest is an underappreciated chapter in the white mold story. Combining an infested field scatters sclerotia across the soil surface and can carry them on equipment to clean fields. Running the combine through an infested patch and then moving to an uninfested area of the same field, or to another field entirely, is one of the fastest ways to introduce white mold to new ground.

Growers managing chronic white mold pressure sometimes harvest infested fields last or clean their combines between fields, though thorough cleaning is difficult in practice. Adjusting the combine to minimize the amount of plant material and chaff left on the surface can help, since sclerotia mixed into heavy residue on the soil surface tend to persist longer than those exposed to weathering. Tillage after harvest that buries sclerotia at least a few inches can accelerate their decay, though as mentioned earlier, the benefit depends on not plowing them back up in a subsequent year. The combination of a two-year or longer rotation with a single deep tillage pass after an infested crop, followed by no-till during the rotation years, is one strategy that leverages the depth-dependent viability patterns researchers have documented.