Scouting for White Mold in Soybean
White mold (also called Sclerotinia stem rot) can substantially reduce yield in soybean. This disease, caused by Sclerotinia sclerotiorum, is especially problematic in fields with dense canopies during early reproductive growth stages coupled with rain, fog, or dew. These conditions create a shaded, moist microclimate conducive to disease development.
Sclerotia – Survival structure that is hard and black with a white interior. Can survive for many years if buried, substantially less in reduced tillage fields. Size ranges from about 1/16 to 1 inch.
Apothecia – Mushroom-like structure growing from sclerotia within the top two inches of soil profile; approximately 1/8 to 1/4 inch in diameter, tan, and cup-shaped. Often form when canopy closes.
Ascospores – Microscopic spores that are released by the millions from apothecia. Spores infect dying flowers.
Mycelia – White fungal growth that spreads from dying flowers. Can move to petioles and stems and may spread to adjacent plants. Eventually mycelia form sclerotia, both inside and outside of soybean plant parts.
White mold disease cycle
Taking accurate notes about where and how much white mold occurs in each soybean field is important for future disease management planning. Tracking disease levels across years will also help determine the potential sclerotia (inoculum) load and the disease risk that may be present in a particular field.
Seasonal and long-term factors favoring white mold risk in soybean include a high yield potential crop with a dense canopy, planting a susceptible variety in a field with a history of white mold, and a history of susceptible crops in the rotation. Factors favoring a dense canopy and white mold risk include early planting, narrow row width, high plant populations, and high soil fertility.
White mold requires a susceptible flowering soybean variety, the presence of the causal pathogen, and conducive weather for disease development.
Check near tree lines or other parts of a field that experience less wind disturbance; par ts of the field with thick canopies; and fields with a history of white mold. White mold often occurs in patches within fields. Within these patches, look for scattered dead plants.
Yield loss is more severe when plants die prematurely or stems are girdled. In addition to causing yield loss, white mold can impact seed quality and reduce grain price because of foreign material at the elevator if sclerotia are present. After harvest, check seed lots for sclerotia and infected seeds. Infected seeds are usually smaller, lighter, white, and cottony.
Symptoms and signs of white mold
Building a management plan based on field history and best disease management practices can help reduce losses due to white mold and minimize sclerotia development for future years. Integrate several management tactics that include varietal resistance, cultural practices, and chemical and biological control products.
Fields at high risk for white mold at flowering stage (R1) may require a fungicide application and possibly a follow-up application at beginning pod stage (R3) in severe epidemic conditions. Some PPO inhibitor herbicides also may be effective at managing white mold.
Choosing resistant soybean varieties is the best method of reducing white mold, especially in conjunction with other best disease man- agement practices. This shows a white mold susceptible variety in front of a resistant variety.
Daren Mueller, Iowa State University; Carl Bradley, University of Illinois; Martin Chilvers, Michigan State University; Loren Giesler, University of Nebraska-Lincoln; Adam Sisson, Iowa State University; Damon Smith, University of Wisconsin; Albert Tenuta, Ontario Ministry of Agriculture; and Kiersten Wise, Purdue University.
Emmanuel Byamukama, South Dakota State University; Anne Dorrance,The Ohio State University; Doug Jardine, Kansas State University; Dean Malvick, University of Minnesota; Samuel Markell, North Dakota State University; Laura Sweets, University of Missouri; Michael Wunsch, North Dakota State University.
Images provided by and the property of the authors, with multiple images provided by Craig Grau, University of Wisconsin-Madison.
The Soybean Disease Management series is a multi-state collaboration sponsored by the North Central Soybean Research Program (NCSRP) through the Soybean Checkoff. This project was funded in part through Growing Forward 2 (GF2), a federal-provincial territorial initiative. The Agricultural Adaptation Council assists in the delivery of GF2 in Ontario. The authors thank the United States Department of Agriculture - National Institute of Food and Agriculture and the Grain Farmers of Ontario for their support. Contributors to this series come from land-grant universities in the North Central states and Canada.
This information is provided only as a guide, and the authors assume no liability for practices implemented based on this information.
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