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Tuberborne Disease Symptoms

On this page
  • Common scab
  • Powdery scab
  • Late blight
  • Silver scurf
  • Black dot
  • Rhizoctonia

Names link to information on identification and management.

Click on photos to enlarge

Common scab
Common scab
Identification tip: Irregularly shaped lesions in patches with dark brown, corky tissue; may appear raised above the tuber surface or as sunken pits.

Powdery scab
Powdery scab
Identification tip: Dark, pimple- or wart-like pustules about 1/8 to 1/4 inch in diameter.

Late blight
Late blight
Identification tip: Brownish purple areas with black margins on tuber surface; underlying rotted tissue is coppery brown, granular, lacking distinct margins.

Silver scurf
Silver scurf
Identification tip: Light brown or grayish, leathery, shiny patches on tuber surface; lesions may turn dark and sooty.

Black dot
Black dot
Identification tip: Gray patches on the tuber surface resemble silver scurf; tiny black dots within lesions distinguish black dot from silver scurf.

Rhizoctonia stem and stolon canker
Rhizoctonia (black scurf)
Identification tip: Hard, irregularly-shaped black flecks in the surface that do not penetrate the skin and are not easily rubbed off.

Statewide IPM Program, Agriculture and Natural Resources, University of California
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