Diseases of Potatoes: Powdery Scab

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 This disease is widespread in almost all of the potato growing areas of the world. Powdery scab is caused by the pathogen Spongospora subterranea f.sp. subterranea, a fungus-like disease of potatoes. This fungus-like disease's closest relative in Canada is the clubroot disease of the cabbage family. Both the clubroot organism and the powdery scab organism produce naked zoospores that need water to move around. The mobile zoospores emerge from cysts that are contained in spore balls. These spore balls are large and easily visible (with a microscope). Spore balls can release swimming spores, that infect potatoes, for up to 20 years in the soil.

Powdery scab infection of potatoes occurs at tuber initiation. They are first visible as small pimples which later become brown. These brown visible pustules are filled with powdery masses of spore balls which in turn contain hundreds of cysts. In some potato cultivars, these powdery spore pustules can be very obvious. Symptoms sometimes develop on tubers after harvest. Early lesions on developing potato tubers are small and purple but if the tubers are set aside for a few days, spore balls will form under these lesions.

Russet varieties of potatoes may not show symptoms of this disease but in some potato cultivars the symptoms are obvious and render the tubers unmarketable. This disease shows up more readily in the wetter areas of the potato field.

Another unfortunate consequence of powdery scab in potatoes is the fact that this fungus-like disease can vector (spread) the mop-top virus. In fact, the mop-top virus survives in field populations of the powdery scab organism. Therefore, if a potato field has a mop-top virus infected powdery scab infestation, then potatoes planted in that field become infected with mop-top.

  • Resistant cultivars to powdery scab - Russet Norkotah
  • Moderately susceptible - Snowden, Atlantic, Superior, Katahdin and Russet Burbank
  • Susceptible cultivars - Yukon Gold and Chieftan
  • Highly susceptible - Kennebec, Norland and Shepody
Photographs and information assembled and prepared for ARD by Dr. Ieaun R. Evans Agri-Trend Agrology Ltd.
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For more information about the content of this document, contact Robert Spencer.
This information published to the web on January 6, 2004.
Last Reviewed/Revised on April 7, 2014.