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Potato tuber blemishes: understanding & diagnosis

Netted scab

Bacteria, Actinomycete

Latin name : Streptomyces spp.

Diagnostic characters for visual diagnosis


  • Tuber infection starts with superficial brown lesions which develop into the regular striking netlike structure typical of netted scab
  • These lesions may be localised, but may also cover almost the entire tuber surface.
  • Netted scab affects all subterranean parts of potato plants. Brown lesions develop on stem bases, on stolons and on roots.



Notes on confirmatory diagnostics   

  • To confirm, several steps must be 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 susceptible varieties (for exemple Bintje or Désirée),
    • Identification  of the  species associated with the lesions by sequencing the 16S rRNA gene or by using specific primers.

Specific references :

Bouchek-Mechiche et al (2000 b). PlantPathology, 49, 3-10

Bouchek-Mechiche et al (2006). IJSEM, 56, 2771-2776.

Loria et al (1997). PlantDisease, 81, 836-846.

Scholte K (2005). Netted scab in: PotatoDiseases, NIVAP, 83-84

more symptoms

Possible confusing symptoms :