Pacific Northwest 1998 An Online Guide to Plant Disease Control

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OSU Extension Office
 
Potato (Solanum tuberosum) -- Bacterial Soft Rot and Blackleg
 
Cause: The bacterium, Erwinia carotovora subsp. atroseptica, is usually associated with blackleg and soft rot in storage while E. c. subsp. carotovora is associated with aerial stem rot, lenticel rot, and soft rot. Moist, cool (below 65° F) conditions enhance blackleg while warmer conditons (70-80° F) are optimal for soft rot. Blackleg can rapidly spread with rain or irrigation and by insects. The principal source of inoculum for blackleg is contaminated seed. The bacteria can be spread among seed pieces by machinery and handling.
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:
  1. Minimize the bacterial contamination in seed tubers (Purchase seed free of bacterial soft rot damage).
  2. Sanitize and disinfest seed-handling equipment.
  3. Avoid injuring seed tubers and allow cut seed to heal before planting.
  4. Crop rotation will help reduce the disease.
  5. Avoid planting in overly wet or dry soil.
  6. Avoid over-irrigation. More frequent irrigation for a shorter time is less favorable for disease.
  7. Do not harvest until tubers are fully mature (after skins "set" and lenticels are closed).
  8. Avoid tuber injuries.
  9. Provide adequate air flow to promote drying, particularly when tubers first enter storage. Avoid packing and storing wet tubers.
Chemical control:
  1. Treat seed with Seed Treatment For Potatoes at 1 lb/100 lb seed.
  2. Firewall at 100 ppm. Soak cut seed pieces in solution for several minutes. 12-hr reentry.
  3. 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:
Koepsell, P.A. 1978. Controlling Bacterial Soft Rot and Blackleg of Potatoes. Oregon State University Extension Circular 954.
Content edited by: Phil Hamm and Cynthia M. Ocamb on January 1, 2009
 
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