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Potato tuber blemishes: understanding & diagnosis

Black scurf

Fungus, Adelomycete

Latin name : Rhizoctonia solani

Diagnostic characters for visual diagnosis

  • Black scurf patches on tuber correspond to compact masses of mycelium called sclerotia (resting form of the fungus) tightly attached to the tuber skin and which are readily rubbed or scraped off. Sclerotia do not penetrate or damage the tuber skin
  • Sclerotia are raised, dark brown to black, irregular in size and shape, on the surface of the tuber. They are ranging from small, flat, barely visible patches to large, raised lumps
  • Brown strands of R. solani mycelium can sometimes be observed around the sclerotia


Notes on confirmatory diagnostics

  • Pieces of  sclerotia  from tubers are cut  and transferred to general isolation media (malt Streptomycine, alcaline water agar, etc..) or selective ones (eg modified Ko & Hora medium), then incubated for 2 to 3 days at 20 °C.
  • Mycelium of  R. solani  is colourless  when young, it becames brown at maturity 
  • Observation of the mycelium under the microscope show hyphae portioned into individual cells by a septum,  often branched at 90° angles and which contain more than 3 nuclei per hyphal cell
  • AG3 is the main anastomosis group (AG's) producing sclerotria on potato tubers, but other AG's may be associated with the disease but are less frequent and less aggressive
  •  Identification of anastomosis group is possible by isolates confrontation and microscopic  observation of the hyphal fusion, sequencing of R. solani ITS, using PCR-RFLP


Specific references:

Campion et al (2003). European Journal of Plant Pathology, 109, 983-992.

Carling D.E & Leiner R.H (1986). Phytopathology, 76, 725-729.

Castro et al. (1988). Phytopathology  ,78,  1287-1292.  

Fiers et al (2011).  Mycologia, 103(6), 2011, pp. 1230–1244. DOI: 10.3852/10-231

Woodhal et al (2008). Plant Pathology, 57, 897–905 Doi: 10.1111

more symptoms

Possible confusing symptoms :