Information on Potato -- Bacterial Soft Rot and Blackleg, An Online Guide to Plant Disease Control, Oregon State University

Pacific Northwest 1998 An Online Guide to Plant Disease Control

Potato (Solanum tuberosum) -- Bacterial Soft Rot and Blackleg

Cause: The bacterium, Pectobacterium carotovorum subsp. atrosepticum (syn. Erwinia carotovora subsp. atroseptica), is usually associated with blackleg and soft rot in storage while P. carotovorum subsp. carotovorum (syn. E. c. subsp. carotovora) is associated with aerial stem rot, lenticel rot, and soft rot. Moist, cool (below 70° F) conditions enhance blackleg while warmer conditions (70-80° F) are optimal for soft rot. Blackleg can rapidly spread with rain or irrigation and by insects. These bacteria can survive on many weedy plants (nightshades, lambsquarters, pigweeds, purslane, etc.). They also reside in tuber lenticels without causing symptoms but can cause disease if tubers with lenticel populations are used for seed. So the principal source of inoculum for blackleg is contaminated seed tubers. The bacteria can be spread among seed pieces by machinery and handling; moving from diseased to healthy seed pieces during cutting.

Symptoms: Blackleg -- leaves curl up; foliage gradually yellows and dies early. At the stem base, mushy light brown to inky black lesions develop. Aerial tubers also may form on stems. Tuber rot usually begins at the stem end and results in a black, slimy rot.

Aerial stem soft rot -- symptoms are in the upper canopy. Lesions range from light brown to colorless. Stems still get very mushy and hollow and may be filled with mucilaginous slime. Often this disease is a secondary infection associated with stem infections by late blight.

Soft rot of tubers -- on tubers, lesions can be as small as a single eye or involve the entire tuber. The rot is extremely soft and colorless. Although rot of the soft-rot bacterium is relatively odor free, secondary organisms usually cause a foul smell.

This potato tuber has soft rot.	Note the black lesion at the stem base and aerial tubers on stems.
Black leg symptoms are seen here on the potato stems.	Soft rot of potato tuber.
Notice the aerial stem soft rot in the center of this photo.

Cultural control:

Minimize the bacterial contamination in seed tubers (Purchase seed free of bacterial soft rot damage).
Sanitize and disinfest seed-handling equipment. Before cutting, hold tubers at 50 to 55°F at 95% relative humidity for 10 to 14 days.
Avoid injuring seed tubers and allow cut seed to heal before planting.
Crop rotation will help reduce the disease.
Avoid planting in overly wet or dry soil; plant when soil temperature is 50°F or warmer.
Avoid over-irrigation, otherwise high disease levels may occur even when using the best quality seed pieces. More frequent irrigation for a shorter time is less favorable for disease. Avoid irrigating with surface water when growing potatoes for seed.
Do not harvest until tubers are fully mature (after skins "set" and lenticels are closed); injuries will be reduced.
Avoid tuber injuries.
Provide adequate air flow to promote drying, particularly when tubers first enter storage. Avoid packing and storing wet tubers.

Chemical control:

Treat seed with Seed Treatment For Potatoes at 1 lb/100 lb seed.
Firewall at 100 ppm. Soak cut seed pieces in solution for several minutes. 12-hr reentry.
Use a disinfectant when washing tubers to prevent lenticel infection. Spray a chlorine disinfectant such as sodium hypochlorite at rates to give 75 ppm available chlorine. (Not recommended in Idaho.) Check water frequently to maintain proper disinfectant level. Dip treatments lose activity rapidly, and the dip solution then spreads the bacteria.

References:
Strand, L. 2006. Integrated Pest Management for Potatoes in the Western United States, 2nd edition. UC Agriculture and natural Resopurces Publication 3316. 167 pp.

Content edited by: Phil Hamm and Cynthia M. Ocamb on January 1, 2010

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