More Reports of Stem and Root Rots of Soybean: Identifying Sclerotinia - UNL CropWatch, Aug 2013

More Reports of Stem and Root Rots of Soybean: Identifying Sclerotinia - UNL CropWatch, Aug 2013

Dead and diseased soybean plants

Figure 1. Often the first signs of white mold (Sclerotinia Stem Rot) will be the appearance of pockets of dead and dying plants in the field. (Photos by Loren Giesler and Amy Timmerman)

August 22, 2013

Stem and root rots continue to be a problem is several Nebraska soybean fields and in northern Nebraska white mold has been reported in several fields. We also continue to see Brown Stem Rot, Phytophthora Root and Stem Rot, and Sudden Death Syndrome. (See CW: Stem and Root Rots Earlier than Normal.)

Soybean stems with Sclerotinia

Figure 2. A white cottony fungal growth on stems is one indication of soybean infection by Sclerotinia Stem Rot.

Sclerotia inside soybean stem

Figure 3. When stems of dead plants with Sclerotinia are split, small black sclerotia of the fungus often will be visible.

White mold, also known as Sclerotinia Stem Rot, is a disease that starts earlier in the season during flowering. The actual infection occurs on the senescing flower which serves as a food source for the fungus. Therefore, all infections in soybean typically start at a node. You can even tell when the infection occurred based on how high up the plant the stem lesions and fungal growth are. The cool wet conditions that occurred during flowering were favorable for infection, and the cool temperatures the last couple weeks were very favorable for fungus growth.

At this soybean growth stage symptoms will appear in individual or small pockets of dead or dying plants. Upon close inspection you will see a white cottony fungal growth on the stems. There also may be dark black bodies (sclerotia) of the fungus on the stems. If it is drier and plants are dead, the stems will be very light (bleached) in color. When dead stems are split, often you’ll see sclerotia of the fungus inside.

Since further spread will be limited to localized spread (no sporulation stages) from plant to plant, management decisions will depend on how widespread the plant infections are. I’ve received several calls regarding whether a fungicide should be applied. While there is no good data on this, if producers are seeing widespread infection of plants across the field, there may be some benefit from treating. Many fungicides are labeled for Sclerotinia Stem Rot and could stop the disease from spreading to adjacent plants. Just keep in mind that the disease will not spread much if temperatures are in the 90s, as predicted for the next week or so. The optimum temperature for growth of this fungus is 75°F.

The best action at this time is to make sure you identify this and other problems correctly so that you can use plant resistance to manage the problem in the future. In general, fungicide applications during flowering to control Sclerotina are often unsuccessful. Soybean genetics will be more consistent; however, no varieties are fully resistant to Sclerotinia. Narrow row spacing also has been shown to increase disease incidence in a field.

As with any disease, correct diagnosis is critical to proper management. If you are uncertain of the cause of damage in your field, I encourage you to have it identified by the Plant and Pest Diagnostic Clinic. More information on these and other soybean diseases can be found at UNL's Plant Disease Central and in CropWatch.

Loren Giesler
Extension Plant Pathologist