Unraveling the Surge of Soybean White Mold in Northern Iowa and Southern Minnesota This Year

Areas in Northern Iowa and Southern Minnesota are facing a significant challenge this year with an outbreak of Soybean White Mold (Sclerotinia sclerotiorum). This fungal disease can have devastating effects on soybean crops. The reasons behind the surge of Soybean White Mold in this region are multifaceted, stemming from a combination of environmental conditions, agronomic practices, and the complex life cycle of the pathogen. In this article, we will delve into the factors contributing to the prevalence of Soybean White Mold in Northern Iowa and explore potential strategies to manage and mitigate its impact on the agricultural landscape.

Environmental Conditions

Photo from University of Wisconsin

The occurrence of soybean white mold is intricately linked to weather patterns, particularly during the flowering and early pod development stages of soybean growth. Despite hot, dry conditions early in the summer, this year’s August weather has favored the development and spread of the pathogen. Elevated humidity levels, prolonged leaf wetness periods, and mild temperatures during the growing season created an ideal environment for the fungus to thrive. These conditions facilitate the germination of fungal spores and their subsequent infection of susceptible plant tissues.

Agronomic Practices

Modern agricultural practices, while essential for maximizing yield, can inadvertently create an environment conducive to the proliferation of Soybean White Mold. Dense plant populations, narrower row spacing, and high canopy density can restrict air circulation within the crop canopy, increasing humidity and leaf wetness.   Some farms this spring had slow emergence due to soil crusting.   A solution was to interplant soybeans in areas, but in some cases, most initial planting emerged, resulting in a very high population of beans in some areas.    Moreover, the application of excessive nitrogen fertilizers, or carry over nitrogen due to dry conditions, can lead to lush vegetative growth, providing the pathogen with more host tissue to infect. This year’s unique combination of factors, including planting decisions and nutrient management, could have unknowingly contributed to the disease triangle.

Pathogen Life Cycle and Reservoirs

Understanding the life cycle of the soybean white mold pathogen is crucial in addressing its prevalence. The fungus overwinters as sclerotia, small black survival structures, in the soil or crop debris. These sclerotia germinate to produce mushroom-like structures called apothecia, which release airborne spores that infect soybean flowers. The disease then progresses through the plant, causing lesions and ultimately leading to yield losses. The presence of previous host crops like sunflowers, canola, or other broadleaf plants in rotation can also harbor the pathogen, increasing its potential for infection.

Photo from University of Nebraska

Integrated Management Strategies

To combat the surge of soybean white mold in Northern Iowa and Southern Minnesota, farmers can implement integrated management strategies that focus on both prevention and control. Here are a few recommended approaches:

Photo from Michgan State University
Photo from University of Nebraska
Photo from University of Wisonsin
Photo from Heads Up
  1. Crop Rotation: Incorporate crops that are not susceptible to soybean white mold in the rotation to help reduce the pathogen’s survival and buildup in the soil.
  2. Varietal Selection: Plant soybean varieties that exhibit resistance or tolerance to white mold.  These cultivars are less likely to succumb to the disease, even under favorable conditions.  Different varieties have levels of tolerance with white mold.  This is true of varieties in every trait platform.  Most times, the varieties less susceptible to SWM will be smaller in structure, allowing more air flow to hold Sclerotinia spores down.  See your AFS seed salesman for recommendations. 
  3. Row Spacing and Plant Density: Adjust row spacing and plant density to promote better air circulation and reduce humidity within the crop canopy.  Narrow rows are great for achieving quicker canopy, but this could limit air movement.  
  4. Fungicide Application: Fungicide application at critical growth stages, such as at or before flowering.  This can help manage the disease.  However, timing and coverage are essential for effective control, and multiple applications may be needed.
  5. Cobra: Lactofen, the active ingredient in cobra, has been shown to help reduce white mold.    This treatment lessens the amount of vegetation but also stimulates the soybean plant to protect itself against infection.   
  6. Contans: Contans is a biological product applied to the soil.   This is another fungus that will feed on sclerotia before it can infect plants.  This  preventative treatment should be made in the fall, prior to the host crop, or  early spring to give Contans adequate time to deplete the Sclerotonia population.
  7. Cultural Practices: Implementing practices that reduce excessive vegetation, such as avoiding excessive nitrogen fertilization, can help prevent lush canopy growth that favors disease development.  Specific fields may also have a history and may be predisposed to more problems with white mold.  Those fields may require practices differing from the standard.  
  8. Seed treatment: Heads Up is a seed treatment that can help suppress white mold in conjunction with other management practices.  Heads Up does not target the sclerotia left behind from the mold but instead works to help enhance the soybean’s natural tolerances to disease. 


The surge of Soybean White Mold for Northern Iowa and Southern Minnesota this year is a complex issue influenced by a combination of environmental factors, agronomic decisions, and the pathogen’s life cycle. While the disease outbreak poses significant challenges to the agricultural community, adopting integrated management strategies can help mitigate its impact and preserve crop yield. By staying informed, implementing best practices, and collaborating with your local Asmus Farm Supply agronomist, we can manage this problem.

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