Pacific Northwest 1998 An Online Guide to Plant Disease Control

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OSU Extension Office
 
Potato (Solanum tuberosum) -- Black Dot
 
Cause: Colletotrichum coccodes, a fungus with a wide host range in many potato-growing regions.
Symptoms: The fungus forms many minute, black sclerotia on senescent potato tissue (including roots, stolons, tubers, and stems) especially toward the end of the growing season, causing root and stolon necrosis. Affected plants' leaves may progressively yellow and wilt. The stem yellows, browns, and dries; then sclerotia appear at the base up to several inches above soil level. Other symptoms, such as dark, necrotic, sunken lesions on stems, petioles, and veins, may occur also. Lesions may advance, causing the affected stem to die back or completely wilt.

Foliar symptoms of black dot are similar to Verticillium or Rhizoctonia diseases.

Notice here the black dot stem infections.

Notice the characteristic tiny, black sclerotia within the infected stems.

A tuber infected with black dot.

Close-up of an infected tuber with black dot.

 
Cultural control:

  1. Do not plant seed potatoes from fields with a history of high incidence of black dot.
  2. Crop rotation (2 to 3 years) may help reduce the inoculum level in the soil.
  3. Carefully manage water and fertilization to avoid crop stress.
Chemical control:
  1. Blocker 4 F at 3 to 7.5 pints/A on 7- to 10-day intervals. Foliar applications of Blocker 4F cannot be made if Blocker or other PCNB products were used in-furrow at planting; a maximum of 2 foliar applications of Blocker 4F/A/year. Do not apply within 45 days of harvest, shorter preharvest interval with reduced rates. For Oregon and Washington only (SLN OR-030015, SLN WA-030014, and SLN ID-030008). 12-hr reentry.
  2. Bravo Ultrex at 0.7 to 1.36 lb/A or Bravo WeatherStik at 0.75 to 1.5 pints/A on 7- to 10-day intervals. Preharvest interval is 7 days. 12-hr reentry.
  3. Heritage at 3.2 to 10.5 oz/A on 5- to 7-day intervals in alternation with other appropriate fungicides. Preharvest interval is 14 days. 4-hr reentry.
  4. Inspire at 7 fl oz/A on 7- to 14-day intervals. Do not apply more than two (2) foliar applications of Inspire before alternating to a labeled fungicide with a different mode of action. Preharvest interval is 14 days. 12-hr reentry.
  5. Quadris Flowable at 6.2 to 15.5 fl oz/A on 7- to 14-day intervals or at 0.4 to 0.8 fl oz/1000 row feet applied in a 7” or less band either in-furrow at planting or as a directed spray at cultivation. Do not apply more than one (1) foliar application of Quadris or other Group 11 fungicide before alternating to a labeled fungicide with a different mode of action. Preharvest interval is 14 days. 4-hr reentry.
  6. Quadris Opti at 1.6 pints/A on 7- to 14-day intervals. Do not make more than one (1) foliar application of Quadris Opti or other Group 11 fungicide before alternating to a labeled fungicide with a different mode of action. Preharvest interval is 14 days. 12-hr reentry.
  7. Quadris Ridomil Gold at 0.82 fl oz/1000 row feet applied in-furrow using 3 to 15 gal water/A.
  8. Revus Top at 5.5 to 7 fl oz/A on 7- to 10-day intervals. Do not make more than two (2) foliar applications of Revus Top before alternating to a labeled fungicide with a different mode of action. Preharvest interval is 14 days. 12-hr reentry.
Content edited by: Cynthia M. Ocamb on January 1, 2010
 
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