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White Mold of Soybean
Symptoms and Signs:
- Grayish-green leaves that later turn brown/tan
- Gray to bleached white lesions on the stem covered in white mycelium in high moisture
- Sclerotia (dark, hardened ball of mycelium) on or in stems and pods
- Seeds may have flat or shriveled appearance
- Survives as sclerotia in the soil or plant debris.
- Sclerotia germinate in spring and summer under favorable conditions, forming either mycelia or apothecia (a cup-shaped fruiting structure).
- From apothecia, ascospores are produced and ejected into the air carried by the wind or splashing water to stems, branches, and pods.
- Initial infection occurs on blossoms, later in the season secondary infections can occur by mycelium infection as it spreads through infected plants.
- Sclerotia can be mixed with seed during harvest where they may spread to other fields.
There is no one tactic that can be used to manage White mold effectively. A combination of cultural, chemical, and biological methods should be used to manage this disease.
Host resistance: Cultivars with high levels of partial resistance are commercially available. Growers should talk with dealers to evaluate which cultivars have the best resistance to the disease.
Cultural practices: Using a long crop rotation of several years of non-hosts such as corn and wheat will minimize inoculum build-up over time. Additionally, tillage is recommended for fields with a first occurrence of this stem rot in order to bury the sclerotia and not pull any back up from below the surface. Sclerotia may survive several years, and can germinate within 5 cm of the soil surface. Reducing the seeding rate is another strategy to reduce humidity and open up the canopy in order to eliminate favorable environmental conditions.
Chemical applications: Chemical applications will not provide complete control of the disease, but fungicides and herbicides can be used to reduce incidence in a field where host resistance is implemented and this stem rot has been a problem in the past few years. See the extended version for a more detailed guideline of how to use chemical applications for control of White mold of soybean.
Biological control: The fungus Coniothyrium minitans has been used to reduce inoculum levels in the field.
Avoid introduction of sclerotia: Sclerotia can survive for many years in the field, so reducing sclerotia introduction is critical. Clean seed-lots, avoid harvesting infested fields until the end of the season to reduce field-to-field spread, and used seed treatments to treat infected seed.