White mold, caused by the fungus Sclerotinia sclerotiorum, is an important yield limiting disease of soybeans in the north central United States. The fungus is endemic to the north central U.S., and infects almost all dicotyledonous plant species. Because of its wide host range, it is an important pathogen of a wide range of agricultural crops including dry beans, sunflowers, canola, potatoes, and all forage legumes.
This web site includes a graphic of white mold's life cycle, and how its life cycle corresponds to plant development. The life cycle graphic is also an image map with several clickable regions, so you may obtain details about a specific area of interest.
Soybean White Mold Life Cycle and Soybean Plant Development
Fully developed Ascospore dispersal occurs at this stage.
Figure 1
White mold infection occurs when ascospores land on soybean flowers, germinate, and form mycelium.
In soybean white mold, the flower petals are necessary for infection and disease development since the flower petals provide necessary nutrients for early mycelium development. Also, the petals probably contain chemicals that stimulate ascospore germination. Shortly after ascospore germination, the fungus begins to break down compounds that hold plant cells together by producing oxalic acid and enzymes. The products of this process are then absorbed as nutrients by the fungal mycelium as the mycelium grows into the plant structures. Damage to plant tissues results, especially in the plant's vascular system; thus, moisture transport is restricted. Portions of the stem above the area of infection then die and dry up. This is when infection is first apparent as dead tissue. This dead tissue appears as dead and dying "flags" that are visible from above the plant canopy.
After flags are visible, the white, fluffy, fungal mycelium that gives the disease its common names white mold and sclerotinia stem rot, appears on stems, leaves, and pods.
Infection can continue throughout the growing season, especially during periods of moderate temperatures from 65 to 75 degrees Fahrenheit that are accompanied by high humidity. As the fungus depletes the nutrients, portions of the fungus coalesce into aggregates of mycelium that become the sclerotia.
Sclerotia are hard, resistant structures surrounded by a layer of mycelium. The mycelium contains compounds that are resistant to fungal or bacterial parasitism. Sclerotia can remain attached to crop residue, fall to the ground, or remain with seed during harvest.
Other compounds control sclerotia dormancy. These compounds are water-soluble and ensure dormancy, and when they are removed by moisture sclerotia germination takes place.
Figure 2: Soybean Field Infested With Sclerotinia Stem Rot
Figure 3: Early Infection
Figure 4: Mycelium and Sclerotia
Soilborne Sclerotia and Seedborne Inoculum
The seasonal development of the fungus causing white mold closely parallels soybean plant development. During winter, soilborne sclerotia remain dormant. As soil warms the sclerotia "germinate", and form stipes that grow toward the soil surface.
When the tips of the stipes are exposed to sunlight, they form mushroom-like apothecia. The apothecia release large numbers of ascospores.
With the release of ascospores there is airborne inoculum present that infects when soybean flowers appear.
Fungus development begins early in the growing season and continues as long as soil moisture is high and near saturation, the relative humidity is high, there is free water on plant surfaces, and when temperatures are between 60 and 75 degrees Fahrenheit.
Figures 5-10
> White Mold Management Practices
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URL: http://www.soybeans.umn.edu/crop/diseases/whitemold/white_mold_life.htm
Last Modified 9/6/05 by:
Sarah Jameson-Jones
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