Soybean White Mold: Effective Strategies for Fungicide Application

Sclerotinia sclerotiorum caused an estimated 11 million bushels of soybean loss in 2022.¹ This pathogen is the cause of the disease white mold, or sclerotinia stem rot. With billions of dollars of damage associated with its name, growers across the North Central United States and Canada are looking for more reliable ways to scout, prevent, and treat soybean white mold infections.  

@Ferenc Somos via dreamstime/Featuring an example of white mold growth on soybean pods. 

Understanding white mold (Sclerotinia stem rot) 

Thankfully, white mold has an extremely appropriate name. If you’ve noticed fuzzy, white, cloud-like growths emerging from deteriorated stems, leaves, or pods, white mold may be the culprit. This fungus is known for threatening soybean yields, but it is one of the most omnivorous plant pathogens in the region, happily feeding on herbs, succulents, flowers (especially sunflowers), and vegetables as well. It can infect plants at any stage of their growth, but it most commonly takes hold in seedlings and fruit. 

@Ferenc Somos via dreamstime/Featuring the effects of white mold on soybean pods and examples of sclerotia. 

Where does soybean white mold come from? 

The fungus that leads to a white mold infection comes from sources in the soil. One of the key characteristics that makes Slerotinia sclerotiorum difficult to eradicate is its ability to leave behind survival structures. These small black sacs are called sclerotia, and they act as resting structures for the dormant fungus, comparable to a seed. Sclerotia are hard and black and can look like mouse droppings, sunflower seeds, or irregular masses. After harvest, these structures overwinter in the field as crop debris, ready to act as sources of infection for newly planted soybeans in spring. 

Unlike some diseases and weeds that are annual threats in soybean fields, white mold can be somewhat unpredictable, making it all the more damaging when conditions are right for it to spread. The pathogen can survive in the soil as sclerotia for up to five years. Furthermore, S. slerotiorum has a broad host range. 

@Ferenc Somos via dreamstime/Featuring a stem carrying sclerotia. 

How does soybean white mold grow? 

Environmental changes act as signals for lifecycle changes in fungi. When the soil is saturated and the maturing soybeans create a full canopy above, the sclerotia grow mushroom-like structures that produce millions of airborne spores almost daily across seven to 10 days. From there, air currents help them attach to nearby stems, senescing flowers, or even drift to nearby sections. 

Infections typically take hold during the R1 to R3 growth stages in soybeans. After infecting the base of the plant, the white mold spreads upward through the plant tissue, invading leaves and stems and leading to aborted pods. Expect disease-affected plants to be coated in the signature white, fluffy mold and to appear brown and withered. The structures will be weak, deteriorate, and shred easily.  

Once mold growth on an infected plant reaches a mature stage, some of the white mycelium (thread-like fungus growth) will turn dark and form blackened, protective coats—new sclerotia. As mentioned above, these sclerotia become time capsules containing everything necessary for the next wave of infection. White mold is a monocyclic disease, meaning that it only infects crops during one period of the year. Once it has completed its lifecycle, it will not infect more plants until the following year. 

📸: BASF internal photograph/Featuring White Mold on soybean pods.

What are the risk factors for white mold? 

When evaluating whether to prepare for a white mold infection in your area this season, calculate your risk based on the following factors: 

• Temperature: Optimal temperatures for development occur when daytime highs are in the 60’s and 70’s. This fungus prefers the cooler temperatures often found in the North Central region of the United States, as well as in Canada. If temperatures are lower than average at the time of flowering, consider scouting for white mold, especially if you have other risk factors. The risk of infection decreases significantly when temperatures are over 85 F.² 

• Soil saturation: Heavy rainfall and waterlogged soil increase the risk of infection from white mold. If you have pockets of poor drainage in the fields, you have another good reason to watch for plants succumbing to white mold. 

• Cultural practices: Narrow spaces between rows (less than 30” apart), high plant populations, and early planting can all increase your risk of encountering white mold. The higher the density of plant matter in your fields, the easier it will be for white mold to find tissue to infect. 

• History of white mold: Controlling white mold requires preventative measures. Fungicides applied to evident symptoms of white mold will have virtually no effect on reducing the disease. Until you have applied fungicide during the R2 stage of growth and see a significant reduction in infection, you can expect to see white mold return under favorable conditions. The sclerotia are incredibly resilient to seasonal environmental conditions and chemical control. Treating the disease before Sclerotinia sclerotiorum reaches the lifecycle stage for sclerotia development is critical. These structures can survive for up to six years or more in soil. A grower might not face changes from the disease every year, but it is likely that if they have had an outbreak once, they will see it again in the same field when the environmental conditions mentioned above are met. 

• Crop rotation: Planting corn or a nonhost crop for two or three years has been shown to reduce the number of sclerotia in soil. Crops like canola, tomato, potato, beans, peas, sunflowers, or cole crops all maintain infection levels and should not be used to control white mold.3 Symptoms of infection can vary depending on the host. For example, tomatoes disintegrate, leaving behind a watery rot. The epidermis of infected beans will slough off easily if rubbed. Tubers near the soil surface may develop small lesions that become depressed areas that enlarge over time, turning black on the surface. In each case, sclerotia develops within the rotted tissue or near the site of the infection. 

• Tillage: As a residue-borne disease, tillage can greatly affect white mold incidence in your fields. The Michigan State University Extension writes that tillage is recommended when managing white mold. However, they note that sclerotia are viable even when buried 8” to 10” under the soil, meaning that tillage may temporarily lower the incidence of infection, but tilling again will resurface these sclerotia. Greater tillage also promotes earlier canopy development, leading to favorable conditions for white mold growth.⁴ 

• Weed control: Many broadleaf weeds act as hosts for white mold, so application of herbicides may help suppress fungus activity in your field. Potential hosts include chickweed, cocklebur, lambsquarters, purslane, ragweed, dandelion, field pennycress, henbit, hemp, prickly lettuce, shepherd's purse, sow thistle, toothed spurge, velvetleaf, venice mallow, wild carrot, wild mustard, wild parsnip, and many more. 

@fotokostic via Canva/Featuring the timely application of fungicide to control white mold. 

Management and fungicide application 

Research from North Dakota State University suggests that spraying fungicide at the R2 stage of growth, coinciding with canopy closure, increases its effectiveness.⁵ Applying fungicide to a full-blown infection has no chance of reducing the infection or saving plants, so if you’ve already spotted this snowy menace tormenting your soybeans, it’s important to start planning your fungicide application for late spring or early summer next season. The good news is that fungicide application isn’t your only method for counteracting fungus. Here’s how to manage white mold in soybeans. 

Scouting for soybean white mold 

In "order" to have a chance at effective prevention, you’ll want to identify white mold before the R2 stage of growth. By the time growers are seeing obvious signs of white mold infection, like flagging or wilting leaves at the top of soybean plants, the window for effective control will have passed. 

As the infection progresses, lesions cause the tops of plants to become grayish-green, wilt, and eventually die. Stems may become soft, watery, and covered with white, moldy growth. Dry, dead stems can have a bleached, white appearance, and hard, black sclerotia can be found on or inside infected stems and pods. 

Fungicides for white mold 

By the time you’re looking at wilting soybean plants in the late R4 or R5 stage of growth, the window for fungicide treatment has passed. Once you know your field is susceptible to a white mold infection, you will want to keep your fungicide at hand. Research shows that applying a fungicide during the R2 stage maximizes your chances of protecting flowers and, ultimately, yield. The same study suggests that your next best window of application is during the R3 stage.⁶ If you’re facing a particularly cool or wet start, conditions will be ideal for white mold, and you should be ready to begin a fungicide treatment. 

• Revytek® fungicide for soybean white mold: This premium fungicide offers uncompromising performance and the proven Plant Health protection. Revytek® fungicide has fast uptake and long-lasting residual effects for more performance without trade-offs. With a +6-bu/A yield increase over the untreated check, this fungicide is a great option for growers prioritizing yield potential. 

• Headline® SC fungicide for soybean white mold: Headline® SC fungicide helps control diseases while improving overall plant health. This fast-acting, broad-spectrum fungicide delivers a high level of activity on more than 50 major diseases that can threaten yield and crop quality. In addition, it helps prevent diseases in more than 90 crops, including soybeans, corn and wheat. 

📸: BASF internal photograph/Featuring close ups of White Mold on Soybean stems.

Harvesting soybean 

If you’ve seen evidence of a white mold infection in your field, make sure you harvest from that field last. Harvesting plants infected with white mold contaminates your combine, increasing the risk that you’ll carry the infection to another field. Be sure to give your combine a thorough cleaning afterward to limit the risk of spreading the fungus. Do not store the contaminated soybean with other crops. 

Soybean row spacing 

Densely planted fields are prime breeding grounds for white mold. To reduce your risk of infection and combat yield loss, consider planting rows at least 30” apart. For particularly troublesome infections, you might also consider reducing your plant population by dropping your seeding rate to further increase the distance between soybeans. 

Consider resistance to white mold infections 

Are you looking to choose a soybean seed that maximizes your chances of fighting off white mold infection? Experts are available to help you make your decisions. Reach out to your seed retailer, a nearby extension office agent, or your regional BASF representative for assistance. 

Sporecaster and Sporebuster 

To help you evaluate your risk of infection in a given field, try Sporecaster. This free phone app, created by the University of Wisconsin, uses aggregated data and predictive modeling to advise you on the likelihood of a white mold infection this season. It’s an easy-to-use tool that can help you decide whether a fungicide application is necessary to protect your yield.⁷ 

Speaking of decisions, another app — Sporebuster — is designed to assist soybean growers in building a fungicide program that is efficient, effective, and most likely to result in a profitable return. Simply input your expected price per bushel, your expected yield and the amount of disease pressure you anticipate from mold. 

@fotokostic via Canva/Featuring thriving soybean rows protected from white mold. 

Soybean and white mold don’t mix 

Protecting your soybean yield is already a balancing act. By taking a few early precautions, understanding your risk factors, and being ready to apply fungicide in the right window of opportunity, you can keep white mold from taking up more of your valuable time and energy. For answers to your questions or additional information, don’t forget to contact your seed retailer, extension office agent, or seed company professional from BASF. 

Learn more at Grow Smart® Live: 

See how we’re helping growers maximize soybean yield while protecting from diseases like white mold: 

• Selecting next year’s soybean varieties 
• Build your plan for white mold 
• Timing your soybean fungicide application amid heavy rainfall 

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Endnotes 

1. “Soybean Disease Loss Estimates from the United States and Ontario, Canada - 2022,” Tom Allen, Daren Mueller, Adam Sisson, Crop Protection Network, 2022. 
2. “Sclerotinia Stem Rot,” Institute of Agriculture and Natural Resources, University of Nebraska–Lincoln, 2023. https://cropwatch.unl.edu/plantdisease/soybean/sclerotinia-stem-rot#:~:text=Sclerotinia%20stem%20rot%2C%20also%20known,hard%20dark%20structures%20called%20sclerotia. 
3. “White Mold,” Iowa State University Integrated Crop Management, 2023. https://crops.extension.iastate.edu/encyclopedia/white-mold. 
4. “Management Recommendations for Soybean Fields Infested with White Mold,” Michigan State University Extension, 2023. https://www.canr.msu.edu/news/management_recommendations_for_soybean_fields_infested_with_white_mold 
5. “Shifting Fungicide Applications for White Mold can Improve Yield,” C. Brown, Soybean Research Information Network, 2020. https://soybeanresearchinfo.com/research-highlight/shifting-fungicide-applications-for-white-mold-can-improve-yield/ 
6. “Optimizing fungicide timing for the management of white mold in processing snap bean in New York,” Pethybridge S., Gugino B., Kikkert J., ScienceDirect, 2019. https://www.sciencedirect.com/science/article/abs/pii/S0261219419302297 
7. “New Smartphone App: Sporecaster, The Soybean White Mold Forecaster,” University of Wisconsin–Madison, 2018. 

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