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Diagnostic characters for visual diagnosis

Presence of fissures on the tuber surface. These take many forms from deep elongated cavities  to short shallow cracks irregular in arrangement

• Rhizoctonia cracks, skin within cracks is usually covered by corky lesions
• Secondary infection by PMTV produces symptoms ranging from shallow reticulate cracks  to deep  cavities accompanied by malformed tubers
• PSTVd infection produces surface cracking, usually parallel to the long axis, tubers may became small, very spindly and elongated
• Other pathological organisms that may cause deformations  include powdery scab, wart disease and root knot nematodes.

Notes on confirmatory diagnostics

The symptom is the reaction of the tuber to stress caused by pathogens

• It may be possible to isolate R. solani  on freshly harvested tuber,  the hyphae of the fungus may still be present

• ELISA/PCR is required to confirm the presence of the viruses or viroids on cracked tubers

• Direct identification of the other causal agents can be achieved by observation of the blemish using stereo microscopy (powdery scb, wart disease, root knot nemetodes.

Specific references:

Turff R (2002).  Fungus more complex than first thought. Potato Review, 12,4-6

Woodhall et al (2011). New Disease reports.23, 31. http://dx.doi.org/10.5197/j.2044-0588.2011.023.031

Tenorio et al (2006). American Journal Potato Reaserch, 83:42

Harrison & Jones (1970). Ann Appl Biol 65: 393-402

Pfannentiel MA, & Slack SA (1980). Phytopathology, 70: 922-926

Crack due to water stress

Crack due to herbicide

Deformation due to abiotic factors