Potato scab

Streptomyces scabies

Potato scab is a common potato disease caused by one of several strains of a bacterium called Streptomyces scabies which lives in the soil. Some strains of scab occasionally affect vegetables such as beets, parsnips, carrots, turnips, rutabaga and radish.

The disease does not affect yield or the storage life of the potatoes. Although scab looks unsightly, the tubers and roots are perfectly edible - simply peel the skin off before cooking.

Symptoms:

  • Potato tuber skins appear diseased with scabby lesions. The scab lesions may be superficial, slightly raised or sunken.
  • The disease affects the potatoes as they develop tubers. Dry soil and high pH promote bacteria growth and infection. Scab tends to do well in light soils high in organic matter. 

Control:

  • Rotate potatoes so that they are not grown in the same area for at least 4 years. Follow potatoes with crops not susceptible to scab such as peas, beans, leafy greens and corn. 
  • Always use certified potato seed. Do not use infected tubers from a previous crop.
  • Potato selection:
    • Good resistance to scab: Norking, Superior, Cherokee and Huron.
    • Intermediate resistance: Chieftain, Russet Burbank, Monona, Norchip, Norgold Russet, Norland, Viking, Avon, Jemseg, Sable, Mirton Pearl and Sebago.
  • Add moderate amounts of organic matter like compost to your soil to help retain moisture. Composted manure derived from animals fed scab-infected potatoes can transfer the bacteria to the soil. 
  • Prairie soils tend to be alkaline which is favourable to the disease. However these soils have a natural buffering capacity that resists changes to pH. For this reason, amending soils to lower pH is neither practical nor recommended.
  • Keep potato plants well watered especially during tuber development. Water deeply (3 cm or 1") once a week. Hill soil around plants as they grow and use mulch in between rows to conserve soil moisture.

 

 

 

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 crops. Retrieved July 7, 2024, from https://www.vegetables.cornell.edu/pest-management/disease-factsheets/potato-scab/