Disease of the Month - Black knot of Prunus

  Hort Snacks - December 2018
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 Causal Organism: Apiosporina morbosa

Crops Affected: all plants within the genus Prunus, including various species of cherries (chokecherry, Nanking cherry, pin cherry, sand cherry, sour cherry, etc.), plums, almond, etc.

Disease Cycle:

  • Ascospores are produced on mature galls in the spring, infecting growing points
  • Infection causes an increase in xylem and phloem cells in the stem
  • The disease will develop internally and externally
  • Galls will grow in size each year, producing spores annually
  • Plant structure is affected and branch strength may be reduced
  • Eventually, plants may die
  • A single infected tree or bush can infect a wide area
  • The most recognizable symptom is the presence of a black tar-like swelling on branches of infected plants
    • Sometimes vulgarly described as resembling "poop-on-a-stick"
  • Initial symptoms include a small swelling or gall at a succulent growing point or fruit spur
  • Swellings grow in size and eventually become ruptured, hard and black
    • Galls may be 6 or more inches in length
    • Galls may also occur at branch junction points and on scaffold limbs
  • Galls may eventually be colonized by secondary fungi, which can reduce sporulation
Swollen tar-like galls of various sizes are visible on branches and branch junctions
Photos by Tricia Simon

  • Remove sources of inoculum in adjacent areas
  • Prune out infected branches during the dormant season, when plants are dormant and galls are highly visible
    • Galls should be pruned out, with cuts being made at least 6 inches beyond obviously infected materials
      • Pruning cuts should be made to an appropriate point (healthy collar, etc.)
  • If possible, pruning tools may be disinfected between cuts, however this may be difficult or impractical in many situations
  • Destroy infected material by burning, burial and/or proper composting, or removal from the site
  • Ensure plants are healthy and free from stress
  • Ensure adequate canopy ventilation
  • Chemical controls are limited, fairly ineffective and expensive
  • Consider hiring a certified arborist for complex pruning activities
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For more information about the content of this document, contact Robert Spencer.
This information published to the web on November 28, 2018.