Disease of the Month - Blackleg of Potato

  Hort Snacks - December 2016
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 Causal Organism: Erwinia carotovora subsp. atroseptica OR Petrobacterium atrosepticum OR Petrobacterium carotovorum var. atrosepticum

New Dickeya Blackleg: Dickeya dianthicola

Crops Affected: potato

Disease Cycle:

  • Bacterial pathogen
  • Typically affects stems and tubers
  • The pathogen resides in the tuber lenticels and in wounds (or on crop debris, in some situations)
  • Favourable conditions at planting, followed by warm weather will trigger disease development
  • Stand loss may occur if seed pieces are decayed, or the disease may move into the plant via the vascular system, in established stands
    • Infection can occur in the soil or during harvest
  • Seed tubers are the primary source of inoculum
    • Other sources of inoculum include infested soil and irrigation water
    • Aerial stem rot comes from bacteria on crop debris, entering through wounds or leaf scars
    • The pathogen is also spread by insects or irrigation water
  • Dickeya blackleg is a more aggressive pathogen than the typical blackleg pathogen
    • It can cause infections at lower inoculum levels, can spread more easily through the vascular system and develops at higher temperatures
  • Plants may have a stiff, upright growth habit and may be stunted
  • Water-soaked black lesions are observed on infected stems
    • Stems have inky black lesions that begin at the rotting seed piece, reaching up the entire stem
    • Lesions darken and merge together, as the infection moves up the stem
      • Vascular tissues can be discoloured, in addition to the decaying pith tissues
      • Plants may be completely rotted at the base of the plant
  • Leaves may turn yellow and leaflets can roll upwards at the margin
    • Leaflets and the entire plant can wilt and collapse
  • Tuber symptoms vary
    • Inky-black sunken lesions develop at the stem end of tubers, with tissues gradually turning grey, then black
    • Black-walled irregularly-shaped cavities extend through the centre of the tuber
    • Lenticels may be sunken (up to inch in diameter) and brown to black in colour
      • The sub-lenticel tissues may be dry and brown
Conditions Favouring Disease Development
  • The disease can be present in both wet and dry weather, although symptoms can vary and the location of symptoms will vary (wet = entire plant; dry = mostly belowground)
  • Typically, warm, moist conditions will allow disease to develop, in field or in storage
  • Cooler, wet temperatures in spring will allow for disease infection, with warm temperatures after emergence speeding up and encouraging further development
  • It is critical to use clean seed, as this will influence initial development
  • Sanitization of any cutting equipment may reduce mechanical disease spread
    • Seed treatment may be appropriate for reducing disease
    • Using whole seed versus cut seed may be beneficial
  • Ensure soil conditions are favourable for rapid emergence and healthy plant growth
    • Avoid poorly drained, cool soils
    • Avoid irrigating prior to plant emergence
  • Encourage rapid leaf drying and reduced canopy densities, if possible
  • Follow a good crop rotation
  • Post-harvest handling
    • Harvest once tubers have a good skin set, once vines are completely dead
    • Avoid bruising and wounding of tubers during harvest and post-harvest handling
    • Encourage wound healing in pre-storage
    • Maintain suitable cold storage conditions
A useful article on Blackleg and Dickeya Blackleg – Dickeya: A New Major Threat to Potato Production in North America – SpudSmart article
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For more information about the content of this document, contact Robert Spencer.
This information published to the web on November 30, 2016.