White mold in soybeans, also called Sclerotinia stem rot, is one of the most damaging diseases in northern soybean-growing regions. It’s caused by a soil-borne fungus that infects plants during flowering and can cut yields by 5 bushels for every 10 percent increase in disease across a field. Recognizing it early and managing the conditions that favor it are the most effective ways to limit losses.
How to Identify White Mold
Symptoms first appear during pod development, not earlier. Leaves on infected plants wilt and turn gray-green before browning, curling, and dying. Because the fungus infects blossoms first, the earliest stem or pod lesions usually show up near colonized flowers, typically 6 to 14 inches above the soil line. Within a few days, those diseased areas turn tan, then bleach out to white. The bleached stems feel pithy and shred easily when you pull them apart.
In humid conditions, infected stems and pods become soft and watery, then get covered in a distinctive white, fluffy fungal growth. That cottony appearance is what gives the disease its common name. As the season progresses, hard black structures called sclerotia form on and inside the stems and pods. They resemble small rodent droppings and are the survival mechanism that keeps the fungus alive in the soil between seasons.
From a distance, affected fields show scattered dead plants standing upright or patches of dead plants clustered together. The disease typically prevents successful pod development on infected stems, so any plant showing symptoms is unlikely to contribute meaningful yield.
What Drives Infection
White mold needs three things to take hold: sclerotia in the soil, a dense canopy that traps moisture, and flowers for the fungus to colonize. Sclerotia near the soil surface germinate and produce small, mushroom-like structures called apothecia, which release spores into the canopy. Those spores land on soybean flowers, which serve as the initial food source. From the flower, the fungus invades the stem and pods.
Cool temperatures and high humidity during flowering create ideal conditions. Predictive models use 30-day averages of maximum temperature, relative humidity, and wind speed to estimate when spore-releasing structures will appear. Canopy closure matters too: the thicker the canopy, the more moisture stays trapped around the lower plant, and the higher the infection risk. Fields that close canopy early, whether from narrow rows or high seeding rates, are consistently more vulnerable.
How Much Yield You Can Lose
The relationship between disease severity and yield loss isn’t linear. At lower levels of infection (25 to 30 percent of plants showing symptoms by maturity), losses are modest, roughly 0.4 to 0.9 bushels per acre. But once disease incidence exceeds about 65 percent, losses escalate sharply. Above that threshold, every additional 10 percent of disease translates to roughly 10 bushels per acre lost. That steep curve means a field can go from manageable to devastating quickly in a wet year.
Canopy Management Through Row Spacing and Seeding Rate
Manipulating the canopy is one of the most practical ways to reduce white mold pressure before the season even starts. Research from North Dakota State University found that intermediate row spacing, around 21 to 22.5 inches, maximized soybean yields in fields with white mold history. Wide rows (28 to 30 inches) reduce disease severity but don’t maximize yields unless white mold pressure is extreme. Narrow rows promote earlier canopy closure and trap more humidity at the base of the plant, which favors infection.
Seeding rate adjustments can help too, but the tradeoff is real. Dropping to 100,000 viable seeds per acre (from 140,000 or higher) reduces white mold pressure, yet it only maximizes yields if at least 15 percent of the canopy ends up diseased by season’s end. If white mold doesn’t show up in a given year, you’ve sacrificed stand density for nothing. This makes seeding rate a bet on disease pressure, best reserved for fields with a strong history of the problem.
Fungicide Timing and Effectiveness
Fungicides for white mold are preventive, not curative. They need to go on during flowering, before symptoms appear. The application window runs from the beginning of flowering (R1) through the beginning of pod set (R3). Once plants reach full pod (R4), fungicides have little value.
Getting the growth stage right matters. To tell R3 from R4, start at the top of the plant and find the first fully open leaf. Count down four nodes from there and check the pod size at that fourth node. If the pod is at least 3/16 of an inch long, the plant is at R3 and still within the spray window. Once that pod exceeds 3/4 of an inch, the plant has reached R4 and the window has closed.
Among fungicide options, Michigan State University research ranks several products as effective for white mold suppression, including those with active ingredients in the carboxamide and benzimidazole classes. Interestingly, the herbicide lactofen (sold as Cobra) often performs as well as the best fungicides under high disease pressure, giving it a dual role in some management programs.
Using the Sporecaster Risk Tool
A mobile app called Sporecaster helps growers decide whether a fungicide application is justified in a given field and year. It uses local weather data and canopy closure estimates to predict whether spore-releasing structures are likely present. Across irrigated and non-irrigated locations, the tool’s predictions during flowering matched end-of-season disease outcomes with about 82 percent accuracy.
The default action threshold in the app is set at 40 percent probability. For varieties known to be highly susceptible, university research recommends lowering that threshold to 20 percent. The tool generates site-specific risk values for three scenarios: non-irrigated soybeans, irrigated soybeans on 15-inch rows, and irrigated soybeans on 30-inch rows. It’s a practical way to avoid unnecessary fungicide passes in low-risk years while catching high-risk situations before the spray window closes.
Biological Control Between Seasons
A naturally occurring fungal parasite can be applied to fields after harvest or the following spring to break down sclerotia in the soil. The parasite attacks and degrades sclerotia within the top 2 inches of soil, reducing the number of spore-releasing structures that emerge the next season. Greenhouse research at Michigan State University showed it cut the formation of those structures by 81 percent and reduced new sclerotia production by 50 percent.
The most widely available commercial product containing this organism is applied as a spray to soil or crop residue at 2 to 5 pounds per acre, then lightly tilled to about 2 inches deep. Deeper tillage is counterproductive because it buries the biological agent below where sclerotia germinate. The product costs roughly $30 to $35 per acre and cannot be tank-mixed with other pesticides or fertilizers, since it contains a living organism. It stores for up to a year under refrigeration or about six months at room temperature.
One important limitation: rotating to another crop the fungus can attack (such as sunflower, canola, or dry beans) right after application undermines the approach. The goal is to let the biological agent deplete the sclerotia bank before the next susceptible crop goes in.
Crop Rotation and Long-Term Soil Management
Sclerotia can survive in soil for several years, which means a single year away from soybeans won’t eliminate the problem. Rotating with non-host crops like corn or small grains for multiple years helps draw down the sclerotia bank over time. Combining rotation with biological control products accelerates that process. Fields with a documented white mold history benefit most from stacking these strategies: wider rows, moderate seeding rates, rotation away from host crops, and biological applications in off-years. No single tactic eliminates the disease, but layering them can keep it below the threshold where yield losses become severe.