Loren J. Giesler, Extension Plant Pathologist
Sclerotinia stem rot, also referred to as white mold, is caused by the fungal pathogen Sclerotinia sclerotiorum. The fungus survives from year to year as hard dark structures called sclerotia. Sclerotia are variously shaped bodies of tightly packed white mycelium covered with a dark, melanized protective coat. Saturated soils and a full canopy favor the emergence of apothecia from the sclerotia. These are mushroom-like bodies that produce millions of airborne spores almost daily over a 7- to 10-day period. These spores are released during favorable weather conditions and can travel to other fields in air currents.
Spores infect plants like soybean primarily through colonized blossoms that are senescing but they can also infect through injured plant tissue. Free moisture must be present on the plant surface for infection to occur. Flowers on the tips of small pods provide a common entrance for the fungus. Invasion of the pod and eventually the stem may lead to lesions covered with sclerotia. During harvest these survival structures are scattered back onto the soil. Thus, inoculum for the next three or more seasons has been distributed.
Initial symptoms are visible during pod development. Leaves will wilt and turn gray-green before turning brown, curling and dying. It is important to observe stems and pods for white mycelium and sclerotia to differentiate Sclerotinia stem rot from other stem and root rot diseases. Since blossoms are infected first, early stem or pod water-soaked symptoms often initiate near colonized flowers. In a few days diseased stem areas are killed and become tan and eventually bleached. This bleached stem will have a pithy texture and will shred easily. Infected plant parts generally will have signs of the fungal pathogen as white, fluffy mycelium during humid conditions and sclerotia on the surface of or embedded in the stem tissue. Although stem and pod infection usually occurs about 6 to 14 inches above the soil line, some basal infection also may be found. Infections will occur after flowering has initiated in the crop.
|White Fluffy Mycelium||Embedded Sclerotia in Stem|
Favorable Environmental Conditions
Disease development and spread will occur from flowering until pod formation. As the flower is directly related to disease development, this disease will only develop if we have wet, humid conditions at flowering with moderate temperatures (<85°F). This is why this is not a consistent problem in most of the Nebraska soybean crop acres.
Soybean varieties vary in their response to Sclerotinia and most companies have ratings in the seed catalog. Avoid planting highly susceptible varieties in fields with a history of this disease. In addition, planting varieties which are short and do not lodge will reduce disease potential.
Row spacing has been shown to influence this disease, with narrow rows resulting in more Sclerotinia stem rot. Fields with a history of Sclerotinia should not be planted in narrow rows. Avoid irrigation during flowering. See the interactive web-based irrigation tool for soybean producers in Nebraska at soywater.unl.edu. The common corn-soybean rotation will not reduce the potential for disease development. Utilizing a longer rotation with corn and wheat has been shown to reduce pathogen buildup and disease risk. As several weeds can be a host for this fungus, it is important to maintain good weed control during rotation years.
Foliar fungicide applications are typically only recommended to be considered in seed fields with a history of severe disease development. Sclerotinia suppressive herbicides can be considered in commercial fields and seed fields with severe disease development history. Results vary significantly in the crop response to any chemical treatments for this disease.
- North Central Soybean Plant Health Initiative Research and Information: White Mold (Sclerotinia Stem Rot)