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.

•  Agristrip test (BIOREBA AG): http://www.bioreba.ch/files/Product_Info/AgriStrip/Sss_AgriStrip_edf.pdf

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

http://www.spongospora.ethz.ch/

Common scab

Skin spot