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