The Sclerotinia fungus forms apothecia, small pink fruiting cups, when soil remains moist for two weeks under the soybean canopy. These fruiting bodies produce thousands of very small spores (ascospores) that colonize decaying blossoms on the soybean plant during periods in which the leaf wetness is high for 12 to 16 hours. This fungus produces oxalic acid, causing plant tissue to collapse, and then the fungus colonizes the dying tissue. The distinct characteristic of this disease is the white fluffy mycelium on the stems. As affected plants age, Sclerotinia forms dark, black, irregular-shaped sclerotia both inside and outside the plant. The sclerotia are then harvested with the seed and are spread back on the soil along with the broken stems by the combine. The sclerotia can survive for five years or more when buried in the soil. This fungus can be introduced into new fields from combines spreading sclerotia or also as infected seed or as sclerotia contaminating seed at planting.
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| Figure 16. Sclerotia with fruiting body (apothecium) of Sclerotinia sclerotiorum. | Figure 17. Fluffy white mold on stems of the fungus Sclerotinia sclerotiorum which causes Sclerotinia stem rot. | |
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| Figure 18. Dark black, rat dropping-like-shaped sclerotia of Sclerotinia sclerotiorum. | Figure 19. Sclerotinia stem rot of soybean caused by Sclerotinia sclerotiorum. |
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