Disease of the Month - Scab of Root Vegetables

  Hort Snacks - October 2017
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 Streptomyces scabies

Crops Affected: carrots, beets, potatoes, turnip, radish, parsnip

Disease Cycle:

  • Scab pathogen is a fungus-like bacterium
  • Persists in the soil for years
  • Typically infects during the early stages of root and tuber development, via the lenticels
  • Colonizes several layers of cells
  • May cause damping off in some situations
  • Symptoms are typical of that observed on potato tubers (most common)
    • Abnormal growth of the host cells results in corky tissues that are darker than the healthy tissues
    • Round, irregular, brown lesions form on the surface – typically less than 1 cm across
    • Scab lesions may be either shallow and superficial or raised and erupting
    • Individual lesions may come together to form a group of scabs
Conditions Favouring Disease Development:
  • Severity and nature of symptoms depends on:
    • Strain of Streptomyces
    • Variety
    • Soil organic matter content
    • Crop rotation practices
    • Weather conditions
    • Moisture availability
  • Dry conditions at or after potato tuber formation can increase incidence, as levels of antagonistic bacteria are reduced; it can be assumed that this would be similar for other crops.
  • Soil pH can influence scab formation – more alkaline soils (over pH 5.5) tend to have a higher tendency towards scab
  • Points of injury (insects, etc.) or immature lenticels are where the pathogen enters
  • Controls are rarely required in most root crops, as scab is superficial and tends to be fairly minimal or limited in these crops
  • Carefully consider crop rotations, maintaining longer rotations if scab is an issue in host crops and ensuring a good separation between host crops, particularly potatoes and other root crops
    • Rotate to include grains and grasses to break the disease cycle
  • Consider adjusting soil pH, either through the use of acidifying fertilizers
  • Ensure soils have good moisture holding capacity and that crops have adequate moisture throughout the growing season
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For more information about the content of this document, contact Robert Spencer.
This information published to the web on September 29, 2017.