Disease of the Month - Common Scab vs Powdery Scab

  Hort Snacks - November 2016
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 Powdery Scab | Common Scab

Powdery Scab

Causal Organism: Spongospora subterranea f.sp. subterranea

Crops Affected: potatoes and some Solanaceous weeds

Disease Cycle:

  • Can be easily confused with Common Scab (Streptomyces scabies) on the tuber
    • Laboratory testing or microscopic analysis is required to positively differentiate between the pathogens
  • Seed and soil borne
    • May survive for many years in the soil (as resting spores)
    • Spread by infected seed, moving infested soil or spreading contaminated manure
  • Lesions develop on roots, stolons and tuber surfaces
    • Galls develop in infected tissues
      • May develop within 3 weeks of infection
      • Contain masses of spore balls, which in turn contain numerous resting spores
  • Resting spores germinate in the presence of plant roots to produce motile, flagellate zoospores
    • Infect root hairs, roots, stolons, young shoots and tubers
    • Pathogen may also penetrate through lenticels or wounds in the tuber
  • Further spore dispersal can occur, resulting in more disease development
    • Multiple generations may occur in a season, via secondary spore production
  • Some sources suggest that spread can occur in storage between infected and healthy tubers through aerial transfer (as spores are physically dislodged from lesions into the air)
  • This pathogen will vector Potato Mop Top Virus
  • Purplish-brown sunken lesions develop on the tuber surface in early stages
  • Brown, raised pustules will develop as lesions mature
  • Pustules continue to enlarge
  • Pustules eventually rupture and release spores
Conditions Favouring Disease Development:
  • Cool, wet soils
  • Poorly drained soils
  • Low-lying or shaded areas of infested fields may also allow development in drier years
  • Initially wet soils, followed by a gradual drying out is thought to favour disease development
  • Avoid planting infected seed
  • Maintain a rotation of at least 4-5 years or more
  • Avoid the use of manure from animals that may have been fed infested tubers
  • Ensure soils are well drained
  • Avoid over-irrigating
  • The use of appropriate seed treatments can reduce spread from infected seed
Common Scab

A.K.A. Scab, Potato Scab

Causal Organism: Streptomyces scabies

Crops Affected: potatoes, carrots, beets, parsnips, rutabaga, turnip, and radish

Disease Cycle:
  • Scab pathogen is a fungus-like bacterium
  • Persists in the soil for years
  • Typically infects during the first 5 weeks of tuber development, via the lenticels
  • Colonizes several layers of cells
  • Does not spread or increase during storage
  • Round, irregular, brown lesions form on the tuber surface – typically less than 1 cm across
  • Scab lesions may be either shallow and superficial, raised and erupting, or deep and pitted
  • Underground stems and stolons may also be affected
  • Almost impossible to distinguish from Powdery Scab
Common scab lesions on a red-skinned potato
Photos by Robert Spencer

Conditions Favouring Disease Development:
  • Severity of symptoms depends on a number of factors
    • Strain of Streptomyces
    • Potato cultivar
    • Soil organic matter content
    • Crop rotation practices
    • Weather conditions
    • Moisture availability
  • Dry conditions at or after tuber formation can increase incidence, as levels of antagonistic bacteria are reduced
  • Soil pH can influence scab formation – unfortunately range for Streptomyces is similar to that of potatoes
  • In other crops, points of injury (insects, etc.) or immature lenticels are where the pathogen enters
  • Use less susceptible cultivar selections if possible (no potato cultivars are resistant)
  • Exercise appropriate crop rotations, with a minimum of 4 years between host crops
  • Protect susceptible crops from injury (e.g. root insects, etc.)
  • Maintain adequate and uniform levels of moisture, particularly around the time of tuber development (typically, tuber formation coincides with flowering on potatoes) – 4-6 weeks after planting
  • Altering soil pH (increasing above 8) can reduce disease severity somewhat, but is difficult to accomplish and potato yield will likely be affected
  • Avoid applying manure from animals fed scabby produce
  • Use clean seed
  • Seed treatments can provide some control of tuber-borne scab, but will not protect daughter tubers from soil-borne scab
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For more information about the content of this document, contact Robert Spencer.
This information published to the web on October 31, 2016.