Pacific Northwest 1998 An Online Guide to Plant Disease Control

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OSU Extension Office
 
Potato (Solanum tuberosum) -- Black Spot (Internal Black Spot)
 
Cause: A physiological disorder associated with low moisture content of the tuber at harvest, bruising, low storage temperatures, and potash deficiency. Bruising increases rapidly when the pulp of potatoes being harvested drops below 47 to 50°F (8.33 to 10°C). Desiccation (pressure flatting) increases black spot when potatoes are handled.
Symptoms: Internal black spots usually are at the tuber's stem end. White, smooth-skinned cultivars are more susceptible.

 
Cultural control:
  1. Maintain a high soil potash level.
  2. Maintain at least 50% of available soil moisture until harvest.
  3. Handle tubers carefully.
  4. Store at 40 to 45°F. Warm to 70°F and hold 2 days before handling. Hold french fry and chipping potatoes at 47 to 50<°F.
  5. Maintain 95% relative humidity to prevent desiccation.
Content edited by: Cynthia M. Ocamb on August 28, 2009
 
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