Potato blackleg is caused by several types of soft rot bacteria. These bacteria overwinter in potato tubers, plant debris and soil. Tubers are invaded through damaged skin. Poor drainage, potassium deficiency, or excess nitrogen can exacerbate potato blackleg.

Susceptible Plants- Potato

Symptoms- Potato blackleg is first seen as small water-soaked lesions on stems. The stems then turn brown or black. 4 in.(10 cm) above and below the soil level, becoming mushy. The plant leaves roll, blister and wrinkle. The lower stem disintegrates and can become foul smelling. Tubers can also be attacked, resulting in gray slimy rot. The potato blackleg disease can affect isolated individual plants and even isolated stems on one plant. Most internal tuber tissues may be consumed by soft rot organisms, sometimes leaving only a shell of skin remaining in the soil.


Cause- Moisture and temperature are the two critical factors in initiation and development of soft-rot diseases. High soil temperatures and bruising of seed tubers favor seed-piece decay and pre-emergence blackleg. Potato Blackleg in growing plants is favored by cool, wet soils at planting followed by high temperatures after emergence.


Prevention and Control- Improving soil structure can reduce the likelihood of potato blackleg occurring from mineral deficiencies. Lift crops during dry weather if possible to prevent cross contamination of healthy tubers at harvest. Treat cut seed pieces with recommended fungicide dressings immediately after cutting.  Use good gardening hygiene. Clean all equipment used for cutting seed tubers thoroughly and then sanitize with an appropriate disinfectant. Avoid damaging tubers. Never save tubers from infected plants for seed.


Harvest tubers only after the vines are completely dead to ensure skin maturity. Low spots in the field should be left unharvested if significant waterlogging has occurred. Maintain proper storage temperatures. Hold newly harvested potatoes at 55-60 F (13-15C) with 90-95% relative humidity for the first 1-2 weeks to promote wound healing.  After the curing period 39F (4C) is optimum for inhibiting new infections of potato blackleg.