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24, chemin de Borde Rouge -Auzeville - CS52627 31326 Castanet Tolosan cedex - France

Last update: May 2021

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Potato tuber blemishes: understanding & diagnosis

Powdery scab

Rhizaria, Phytomyxea

Latin name : Spongospora subterranea

Diagnostic characters for visual diagnosis

  • Purplish-brown pustules, usually circular, of 0.2 to 5 mm in diameter, but sometime wider and deeper, initially developping  on tuber surface and extending under the periderm to form raised lesions
  • The lesions become filled up with a dry, powdery mass of sporeballs
    At maturity, these lesions release the spore balls and become partly or entirely empty
  • As pustule bursts and reveals powdery sporeballs, small flaps of loose skin remain at the edge of the lesion
  • Sometime, atypical symptoms with brown necrotic tissues may be observed under the periderm
  • Lesions can expand in both directions, depth and width, to form deeply eroded cankers and outgrowths that induce mishapened tubers
  • Roots and stolons of the plant can be infected leading to the development of white galls (turning black when exposed to the air) of 1 to 10 mm in diameter


Notes on confirmatory diagnostics

  •  Microscopic observation:
    • Place the powder scraped from the lesions  on a microscope slide with cotton blue, and observe the presence of typical sporeballs, at 10 and 40X magnification.


Specific references:

Bouchek-Mechiche et al  (2011). EuropeanJournalofPlantpathology, 131, 277–287

Merz U & Faloon RE (2009). PotatoResearch 52, 17-37

Van de Graaf et al (2003). European Journal of Plant Pathology, 109, 589-597

more symptoms

Possible confusing symptoms :