Potato Scab - Frequently Asked Questions

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 What is Potato Scab?
Common scab of potatoes is a bacterial disease, caused by the pathogen Streptomyces scabies, which, in some regards resembles a fungus. Symptoms include tan to dark brown, rough-textured lesions on the tuber surface, typically less than 1cm across. Scab is typically introduced into the soil by infected tubers, and will survive indefinitely in the soil. Infection typically occurs during the first 5 weeks of tuber development, through the lenticels on the tuber surface. Common scab is most severe in warm, quick-drying soils and increases through a pH range of 5.2 to 8.0.

Red Norland potato infected with common scab of potato
Photos by Robert Spencer

What is the economic impact of potato scab?
Common scab is a cosmetic disease, with no effect on yield. Scab is more of a problem in table potatoes than in processing potatoes, as scab lesions are restricted to the tuber surface and peeling removes the problem. Severe scab can reduce the quantity of useable product, as more peeling is required. Common scab does not spread between stored potatoes.

Will soil amendments reduce scab incidence and severity?
It is theoretically possible to adjust soil pH to outside the pathogen’s preferred range. In reality, the quantity of amendment required to accomplish a change in pH is considerable and cost prohibitive. If the necessary adjustments were accomplished, the potato crop itself would be outside of its preferred range and yields may suffer.

What can I do to control scab?
Scab cannot be eliminated but incidence and severity can be reduced through a combination of practices.
  • Avoid introducing scab into soil by planting scab-free or treated seed
  • Rotate to other crops for 3-4 years between potato crops
  • Avoid susceptible crops in the rotation (e.g. root crops, such as carrots, beets, rutabagas, turnips, etc.)
  • Green manuring (rye, millet, oat) has been reported to reduce the incidence of scab
  • Maintain adequate and uniform soil moisture during the time of tuber formation and growth (tuber initiation starts 4-6 weeks after planting)
  • Plant more resistant cultivars

What cultivars of potato are less likely to develop scab?
Resistance to scab varies considerably. Historically, certain types of potatoes (e.g. russet types) tended to be less susceptible than other types. Breeding work has resulted in a decent number of more resistant / less susceptible varieties, in all colours. A table outlining the resistance rating of a number of registered potato varieties in Canada is below.

Potato Scab Disease Resistance Rating Table

Prepared by Robert Spencer, Ag-Info Centre, Alberta Agriculture & Forestry

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For more information about the content of this document, contact Robert Spencer.
This document is maintained by Brenda McLellan.
This information published to the web on June 2, 2003.
Last Reviewed/Revised on September 11, 2017.