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Potato tuber blemishes: understanding & diagnosis

Common scab

Bacteria, Actinomycete

Latin name : Streptomyces spp.


Diagnostic characters for visual diagnosis

Three main types of cork-like lesions including shallow, raised and deep pitted lesions

  • Shallow  scab: superficial, roughened lesions  often slightly invading below the neighbouring healthy skin
  • Raised or erumpent scab: lesions are slightly raised above surrounding peridermal tissue. They may coalesce forming large scabby areas
  • Deep or pitted scab: craters which may be 1 to 10 mm deep, roughly circular or sometimes star-shaped lesions, up to 10 mm in diameter


Notes on confirmatory diagnostics

To confirm the diagnostic, different steps must completed because some saprophytic strains might be isolated together with the pathogenic ones :

  • Isolation of Streptomyces strains on water agar or on other semi selective medium,
  • Selection of the pathogenic strains by testing their pathogenicity on radish (radish is susceptible to common scab, the test is reliable, easy and quick) or/and amplifying the TaxAB gene present only in pathogenic strains
  • Identification  of the species associated with the lesions by sequencing the 16S rRNA gene or by using species-specific primers


Specific references :

Bouchek-Mechiche etal (2000 b). Plant Pathology, 49, 3-10

Lehtonen et al (2004). Plant Pathology, 53(3), 280–287

Wanner, L.A (2004). Plant Disease, 88, 785-796

Wanner, L.A (2006). Phytopathology, 96, 1363-1371

more symptoms

Possible confusing symptoms :