Fungal Diseases in Soybean

Figure 1

Figure 1. SDS leaf and stem symptoms.

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Foliar symptoms of brown stem rot (BSR), and sudden death syndrome (SDS) can look similar and include chlorosis and browning of the tissue between the veins (Figure 1).  Wet weather and moist soil conditions favor disease infection and development. These foliar disease symptoms may also look like early crop maturity and drought stress. There are a few ways to distinguish among fungal soybean diseases.

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Figure 2

Figure 2. BSR browning of the pith.

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Distinguish between SDS and BSR by splitting the stem and looking at pith coloration. The pith will be discolored with BSR while the pith will be white with SDS (Figure 2). SDS usually shows up in spots or scattered areas throughout the soybean field. The disease generally becomes visible as the soybean plant starts to develop pods. Stem canker causes external reddish brown stem lesions, whereas SDS and BSR will not. Charcoal rot is best diagnosed by scraping the upper tissue of roots and lower stems and looking for gray discoloration and black spots (microsclerotia) inside (Figure 4).

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Figure 3

Figure 3. Mycelial growth of white mold on soybean stem.

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White mold produces white, cottony mycelial growth on the outside of the stem and pods, wilted leaves, and stems that appear “bleached�? and shredded. Sclerotia are small black structures found on and inside plants that have been infected by white mold. White mold is the only disease discussed here that has in-season management options.

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Figure 4

Figure 4. Charcoal rot microsclerotia in stem tissue.

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Seasonal risk factors favoring white mold disease development include: cool temperatures, normal to above normal precipitation and soil moisture, early canopy closure, a history of white mold, and soybean product susceptibility.