Field Crop Pathology

White Mold


White Mold

Page Author: Audrey Conrad
Last Updated: 10/8/20

Pathogen

White mold in soybean is caused by the pathogen S. sclerotiorum. S. sclerotiorum is a fungus that overwinters in the soil in the form of sclerotia which are small black survival structures, usually about the size of a grain of rice. Infection is highly influences by environmental conditions. The sclerotia germinate in the spring during cool and moist conditions producing ascospores which infect the plant through a natural opening or wound. The pathogen growths throughout the plant creating water soaked lesions that cause premature senescence of leaves and in extreme circumstances premature death of the whole plant. White mold can be found throughout the soybean growing region of the central United States and Canada.

Disease Symptoms and Signs

Symptoms

Plants will typically appear healthy until beginning reproductive stages. Early white mold symptoms include premature senescence of leaves and in extreme circumstances premature senescence of the the entire plant.

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Description: Field displaying moderate white mold symptoms. Plant displaying typical white mold symptoms of wilting and premature senescence of leaves.

Signs

Occasionally apothecia, which are small tan cup shaped mushrooms, can be spotted in the field early in the growing season and are a sure sign that white mold is active in that field. After flowering occurs, the first sign of white mold infection is fluffy white mycelial growth on the main stem or petioles. Black sclerotia will begin to develop on the surface of the infected tissue as the fungus matures.

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Description: Typical white mold signs include fluffy white mycelium growth on stem and petioles with sclerotia developing as the fungus matures.

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Description: Sclerotia of various shapes and sizes collected from infected plant in northeast Indiana. Approximate size of sclerotia.

Favorable Environmental Conditions

Cool (50 to 68 degrees F) and wet (-0.01 to -0.3 MPa) conditions early in the growing season is favored for sclerotia germination. Canopy closure as well as the presence of morning dew or regular irrigation can further promote disease pressure. Consistent rain or irrigation after the canopy has closed will further promote disease pressure later in the growing season. Hot and dry conditions during the early season and into beginning reproductive stages is not conducive to white mold infection.

Disease Management

No soybean variety is completely resistant to white mold. Certain varieties have moderate tolerance and should be used in fields with a history of white mold. Other cultural practices can be used to further reduce white mold disease pressure.

-Variety selection
-Wide row spacing (30")
-Lower planting populations
-Proper irrigation rates
-Rotation to a non-host crop such as corn or wheat
-Tillage*
-Timely fungicide or biofungicide application

*Historically it was believed that tillage would help to burry sclerotia in the top 2-inches of the soil reducing white mold disease pressure. It is now understood that tillage might create favorable conditions for the development of white mold. Dormant sclerotia buried deeper in the soil could potentially be brought to the surface and into favorable conditions for germination.

A timely fungicide or biofungicide applications at beginning reproductive stages might help to control disease pressure. Current research is underway to better understand white mold management practices in soybean with fungicides and biofungicides in Indiana.

Additional Resources

The sporecaster app is a tool developed by the University of Wisconsin to assist in making management decision for white mold. Available for Apple and Android.

Further Reading