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:

• This is a fungal pathogen
• Spores (ascospores) are produced on mature galls in the spring
  • Spores are blown from the galls to infect growing points on host plants
    • These growing points can be at the ends of branches, as well as at axillary branches like fruiting spurs, etc.
  • Initially, infection causes an increase in xylem and phloem cells in the stem, leading to swelling
    • Initially, the swellings are small and inconspicuous, but increase in size rapidly
    • The galls will increase in size each year, and will continue to produce spores each year
  • The structure of the plant is affected as the galls develop
    • Branch strength may be reduced as the galls girdle the branch
  • Eventually, plants may die
  • The fungus will develop and spread both internally and externally
  • A single infected tree or bush can infect a wide area

Symptoms:

• 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 increase 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 structural/scaffold limbs
• Galls may eventually be colonized by secondary fungi, which can reduce sporulation

Management:

• 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 (15cm) beyond obviously infected materials
    • Pruning cuts should be made to an appropriate point (healthy collar, etc.), to avoid creating an additional weakness
    • Theoretically, pruning tools may be disinfected between cuts to avoid introducing spores or pieces of the pathogen into the cuts, however this may be difficult or impractical in many situations
  • Destroy infected material by burning, burial and/or proper composting, or removing galls from the site
  • Ensure plants are healthy and free from stress
  • Ensure adequate canopy ventilation
  • Chemical controls are limited, mostly ineffective, and expensive
  • Consider hiring a certified arborist to carry out complex pruning activities