A Black Knot infection as seen from the ground. Note the characteristic tar-like swelling and how the fungus grows to envelop the entire branch.

The builders responsible for many of the homes in our community choose to plant trees from the Prunus family. The Schubert Coke Cherry in particular, is common throughout Somerset. Unfortunately, these trees are highly susceptible to Black Knot (Dibotryon morbosum). Now that your trees have dropped their leaves, it may be a good idea to check them for signs of infection. A recent visual survey of the community reveals that many of the neighbourhood Coke Cherries are exhibiting signs of the disease.

Black Knot will encircle and kill branches if left unchecked, and provide habitat for insects. The good news is that it is usually not fatal to the host, and can be controlled through pruning.

Per Alberta Agriculture, you can remove infections by doing the following:

  • 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

Once removed, the diseased branches should be immediately placed, and sealed, in plastic garbage bags to prevent spores from spreading prior to disposal. As per the Alberta Agriculture website, “Diseased knots can produce and release spores for up to 4 months after removal.”

Charles Clark, Somerset Residents Association

Recommended Hyperlinks for Further Reading:

Alberta Agriculture and Forestry – Management of Black Knot

Battleford News Optimist – Black Knot Disease

Calgary Herald – The Inevitable Schubert Choke Cherry

999 Somerset Dr. SW
Calgary, AB
P: (403) 807-8736
F: (403) 201-5565

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