WSU Extension


Potato : Rhizoctonia canker (Black scurf)
(revision date: 6/3/2014)

Rhizoctonia canker is caused by a fungus commonly found in the soil. On potatoes, young underground tissues may be attacked. The reddish-brown to brown lesions may result in girdling of sprouts, stolons, or roots. A powdery, gray coating of fungus may be seen on the stems near the soil line. Infection may also result in leaf curl on more mature plants. Tissues become more resistant to infection as they mature and as they emerge from the soil. Tubers develop "scurfy" patches of hard black fungal structures on the skin. These patches do not extend into the tissues, but are difficult to remove. The fungus may also cause a mild netting or scurf of the skin without forming the black structures. Tuber symptoms are often more severe in soils high in organic matter. Aerial tubers commonly form on infected plants.
Management Options

Non-Chemical Management
  • Plant disease-free seed potatoes.
  • Rotate crops. Do not plant potatoes in the same location each year.
  • Plant in warm, well-drained soils. Delay planting until soil is warm, if necessary.
  • Shallow planting speeds up shoot emergence from the soil and decreases infection. Hill up plants gradually after shoot emergence.
  • Avoid overwatering.
Select non-chemical management options as your first choice!

Chemical Management

None recommended

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Caption: Potato black scurf fungal structures
Photo by: R.S. Byther
Caption: Potato black scurf symptoms on stolon
Photo by: R.S. Byther