Common Scab of Potato

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

Symptoms:

• 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

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, the range for Streptomyces is similar to that of potatoes
  • Changing pH to combat the pathogen (if possible) can reduce the severity, but may reduce the productivity of the potato plants at the same time
• In other crops, points of injury (insects, etc.) or immature lenticels are where the pathogen enters

Management:

• Use less susceptible cultivar selections if possible
  • There are cultivars that have a range of resistance levels, along with different qualities
  • Canadian Food Inspection Agency – Canadian Potato Variety – descriptions
• 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
  • Commercial seed treatments can provide some control of tuber-borne scab, but will not protect daughter tubers from soil-borne scab