Managing Black Knot

What is Black Knot?

Black knot disease is caused by a fungus and affects a wide range of prunus species including plums, cherries and apricots in Alberta. The disease is extremely aggressive on mayday and Shubert chokecherry trees. The disease has progressed over the last decade and is now causing significant problems with trees.

How can you recognize Black Knot?

The most distinguishing symptom of Black Knot is the characteristic black, tar-like swellings that develop on branches of the infected plant.

What can be done to control Black Knot?

• Removal of sources of inoculum (prevents population build up)
  • Prune out all knot-bearing branches during late fall, winter or very early spring when plants are dormant and knots are easy to see
  • Remove infected branches to at least 15-20 cm (6-8 inches) below knot. NOTE: It is preferable to prune an infected branch further back to an appropriate location, such as a healthy collar, rather than leave a stub
  • As a precaution, cutting blades should be cleaned and disinfected after pruning, if possible, especially if cuts have been made through obviously infected material
  • For knots on scaffold branches or trunks that can’t be removed, cut away diseased tissue down to good wood and at least 1 cm (1/2 inch) beyond the edge of the knot
  • Failure to remove branches beyond the internal growth will result in re-growth of the fungus
  • DISEASED WOOD MUST BE DESTROYED IMMEDIATELY (burned, buried or removed from site). Diseased knots can produce and release spores for up to 4 months after removal. Proper composting can help to accelerate the breakdown of infected materials
• Ensure plants are healthy and free from stress (not a guarantee from disease)
• Regular monitoring
• Ensure adequate canopy ventilation through proper pruning
• Chemical control (preventative not curative)
  • Few choices available
  • Not usually recommended unless for valuable plantings, such as collections, orchards, arboreta or for severe infestations

How do I dispose of my cuttings?

• Immediately place your cuttings in a plastic bag and place the plastic bag in with your regular solid waste.
• Do not place the plastic bag with the cuttings beside your garbage bin.

You can visit Government of Alberta Agriculture and Rural Development  to learn more about Black Knot.