Managing Black Knot


What is it?

Black knot is a fungal disease that causes an infected plant to develop greenish brown to black swellings. The disease affects a wide range of prunus species, including plums, cherries and apricots in Alberta. Black knot is extremely aggressive in, but not limited to, Mayday and Chokecherry trees.

Why is it a problem?

Black knot reduces plant growth and deforms tree branches. Trees that are heavily infected become stunted and may eventually die. Its spores are released following periods of warm, wet weather and are spread by birds, insects, splashing water and through the wind.

How to identify black knot

While it is important to regularly monitor your trees for black knot symptoms, it is easiest to identify between late fall and early spring when plants are dormant and their branches are bare. Typically, black knot will first appear as small greenish brown swellings on plant stems. As the knots mature, the swellings will become black and hard.

What to do if you detect black knot

  • Prune the infected branches, preferably between late fall and early spring when the plant is dormant and the knots are easier to see.
  • Remove the infected branches to 20 centimetres (8 inches) below the knot. It is better to prune an infected branch to a healthy collar, rather than a stub.
  • Sterilize cutting tools with bleach between each cut to prevent further spread of the disease.
  • Safely dispose of infected branches immediately. Once removed, diseased trimmings can continue to produce and release spores for months.

How to dispose of infected cuttings

  • Immediately place your cuttings inside a plastic bag in your black waste cart and put out for regular waste collection. Ensure your waste cart lid is fully closed.
  • Do not mix infected cuttings with other organic materials, including those in your green organics cart.
  • Infected cuttings are considered waste and accepted at the Eco Centre. Cuttings but must be in a sealed plastic bag. Disposal fees apply.

How does the City manage public trees that are infected with black knot?

The City of Spruce Grove regularly inspects public trees for black knot. The City prunes infected public trees in the late fall, winter and early spring. The general public is not permitted to damage, disturb, remove, move, cut or prune public shrubbery or a public tree.