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

How to dispose of infected cuttings

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.