No. 22/August 28, 1997
Sclerotinia Stem Rot
Sclerotinia stem rot is caused by the soilborne fungus Sclerotinia sclerotiorum. The disease is usually a minor one in Illinois, except for local outbreaks (usually where snap beans, canola, or sunflowers have recently been grown) during prolonged wet periods. The disease is most common in areas of fields where air circulation is poor, such as, near woods. The first symptoms, often observed on older plants, are the wilting and withering of the upper leaves. A white, cottony growth appears on the branches, pods, and stems of the soybeans, usually near the soil line and originating at stem nodes. Large survival bodies (sclerotia) that are round to irregular, and eventually hard and black, are formed on and inside the stem, occasionally, in the pods. These sclerotia may be partly covered with the dense cottony fungal growth. Plants die prematurely when the stems are girdled by the fungus. The withered leaves remain attached to the stem for some time. Pod development and podfill above the girdling stem lesions are greatly reduced. Soybean seedlings may be killed before or after emergence by a watery, soft rot. Seeds may become infected within diseased pods. Infected seeds are discolored, flattened, and smaller than healthy seeds and sometimes replaced by black sclerotia.
Sclerotia of the Sclerotinia fungus can survive in the soil for long periods and are highly resistant to most fungicides. The sclerotia germinate within 2 inches (5 cm) of the soil surface by producing one to many light tan to brown, funnel-shaped structures (apothecia) during prolonged periods of cool (40° to 59°F, 5° to 15°C), wet weather. Large numbers of asci are formed in the apothecia, which literally eject "clouds" of ascospores under proper conditions. The windborne ascospores germinate and infect soybean blossoms, stems, branches, and pods under very damp conditions.
- Do not rotate soybeans with garden beans, snap beans (Phaseolus spp.), canola, or sunflowers. Control broadleaf weeds that may serve as hosts.
- Thoroughly clean contaminated seed lots to screen out some of the sclerotia. At a foreign port of entry, even a few sclerotia in a shipment intended for human consumption are grounds for rejection.
- Grow soybean cultivars that do not lodge readily.
- Avoid planting soybeans in narrow rows (less than 30 inches or 76 cm) in fields with a history of Sclerotinia stem rot.
- Avoid irrigation at flowering. High humidity in the canopy at this time increases disease levels.
H. Walker Kirby, Department of Crop Sciences, (217)333-8414