Stem canker and black scurf (Rhizoctonia solani) are a kind of potato disease which causes the appearance of black patches on the surface of tubers.
Tuber symptoms include black patches on the surface of tubers which can be rubbed or scraped off, and strands of brown fungal material (mycelium) which can sometimes be spotted around the black scurf when viewed under a magnifying glass.
In cases where the plant’s stolons and underground stems are severely infected with the disease, there may be the appearance of small, aerial tubers above ground.
Stem canker is the most damaging part of the disease, and is difficult to detect as it occurs underground.
Black scurf is caused by the fungus Rhizoctonia solani, which is both seed and soil borne and can exist in the soil even in the absence of potatoes.
There are no potato varieties which have a particularly good or poor resistance to black scurf or stem canker.
Cultural and Methods:
Black scurf and stem canker can be avoided by using only well sprouted seed, avoiding early planting during cold conditions and by avoiding heavily infected seed.
Chemicals have only a very minimal effect on soil borne instances of the disease, however fungicides iprodione, pencycuron and tolclofos-methyl may, when applied to the seed, reduce instances of stem canker and black scurf in seed borne cases.