Rhizoctonia black scurf

Rhizoctonia spp.

Often referred to as “dirt that won’t wash off”, Rhizoctonia is a fungal disease of potatoes.  The disease is caused by a fungus in the soil that grows on the surface causing irregular hard small black spots of the potato tuber.  The incidence of the disease tends to be higher in the weeks following planting when the weather is cold and wet, when plant growth is slower relative to the growth of the fungus.

The spots look unsightly, but have no effect on the inside flesh of the tuber.  The potato is edible and the spots can be easily peeled off before cooking.

Symptoms:

  • Small hard, black spots on the potato skin.
  • Reddish-brown to brown lesions may develop on sprouts, stolons and young stems.
  • Secondary symptoms include a mat of white to gray mycellium at the base of the stem that can be easily rubbed off. 
  • In severe infections, leaves may roll upward and be reddish in colour.

Control:

  • Rotate where you grow potatoes so that they are not grown in the same place for at least three years to avoid a build-up of the disease.
  • Purchase certified seed every year.
  • Adding 3 - 5 cm of organic matter to the soil such as compost, which tends to reduce black scurf.
  • Plant later in spring and more shallowly than you would normally into warm soil (above 8°C). This speeds up emergence of the plant and reduces the risk of stem canker.
  • Harvest as soon as the crop is mature as the severity of symptoms increases rapidly once the tops begin to die back.
  • Remove all debris from garden after harvest as the fungus over-winters on infected plant material. Debris can be hot-composted.

 

 

Sources:

Howard, R. J., Seaman, W. L., & Garland, J. A. (Eds.). (2002). Diseases and pests of vegetable crops in Canada. Vancouver, British Columbia: The Canadian Phytopathological Society and the Entomological Society of Canada.

McGrath, M. (2021, April). Vegetable MD Online. Retrieved July 7, 2024, from https://www.vegetables.cornell.edu/pest-management/disease-factsheets/rhizoctonia-canker-and-black-scurf-of-potato/