Sclerotinia Stem Rot (Soybeans)

White Mold

Sclerotinia sclerotiorum

Soybean plant stems infected by Sclerotinia Stem Rot
Soybean plant stems infected by Sclerotinia Stem Rot. Photo courtesy of Daren Mueller. Licensed under CC BY 3.0.
Sclerotinia stem rot is a significant factor in yield loss throughout North American soybean fields. It is characterized by its white mold that grow on the stem, leaves, and pods of infected plants. It can also be quickly recognized by its sclerotia (inactive bodies of the fungus) that resemble mouse or rat droppings and are located on the outside of the stems and pods.
White mold survives in the soil throughout the winter and generally infects the plant during the flowering stages through its flowers. It is favored by cool, wet conditions where the fungus forms small mushrooms on the surface of the soil that release spores. The infection begins when the spores land on the surface of the flowers of soybean plants.
Symptoms of sclerotinia stem rot are not usually noticeable until August when the plant's stem begins to develop gray or white lesions at the nodes. These lesions quickly progress up and down the plant, sometimes covered in fluffy, white mold. Black sclerotia will appear on the stem and pods, resembling mouse or rat droppings. The plant will have by this point begun to wilt, which in some cases will result in lodging.
Black sclerotia produced by sclerotinia stem rot
Black sclerotia produced by sclerotinia stem rot. Photo courtesy of Daren Mueller. Licensed under CC BY 3.0.
Overall impact of sclerotinia stem rot is estimated at 5-10 bu/acre decrease for every 10 percent incidence in soybeans. It has in recent years been considered as one of the most dangerous soybean diseases, causing an estimated $560 million in losses for farmers in 2009. This disease impacts yield in both quality of seed and number of seeds produced.
Rotating non-host crops for 2-3 years at a time will reduce sclerotia levels in soil. Tillage can be effective, but overall evidence for effectiveness is inconclusive. No-till fields are thought to be more susceptible to disease. Weeds can be hosts for this pathogen, so it's important to minimize weed populations in and around soybean fields. Other measures taken have been increasing row spacing, decreasing number of plants per acre, and planting later in the spring.
Fungicides currently on the market do not offer complete control over sclerotinia stem rot, but they do maintain partial control. They are best used in combination with mechanical and/or cultural control measures. Active ingredients listed for compression and/or control are thiophanate methyl, boscalid, tetraconazole, and prothioconazole. Chemical applied at R1 growth stage has proven more effective than R3. Please contact your local plant pathologist to learn what is most effective in your area and how to use it best. Always read and follow all label instructions.
Though there may not be totally resistant varieties available, be sure to select varieties that do have some resistance to sclerotinia stem rot. Planting cover crops will also provide the potential for seedlings to emerge sooner, thus be less susceptible to infection (earlier flowering). There are certain biological control measures that can be taken. Please contact your local plant pathologist for proper recommendations.