Black Knot

What is Black Knot?

Black Knot is a fungal disease that causes infected plants to develop black tarlike swellings. The disease affects a wide variety of trees of the prunus species including, but not limited to plums, cherries and apricots. The disease is also very aggressive in Mayday and Chokecherry trees.

A recent survey in Alberta revealed a significant and widespread distribution of Black Knot found in commercial, municipal, private and natural plantings. This disease reduces the aesthetic value of affected specimens, as infections spread rapidly; high levels may result in the eventual death of the plant.

How to identify Black Knot?

Black Knot is distinguished by the black, tar-like swellings that develop on the branches of the infected plant or tree.

Initially, a small, olive-green gall or swelling will develop at a succulent growing point or fruit spur (as a result of spores landing and infection taking place). This swelling will grow until it is mature after 2-3 years. The mature galls are hard, black, 10 to 15 cm (4 to 6 inches) and may be somewhat ruptured. Mature galls will produce and release a vast amount of spores during the bloom period, resulting in a rapid increase in infections. The fungus continues to grow internally and externally, with the branch eventually becoming girdled and dying.

What is Devon doing to manage Black Knot?

Each year, Devon’s Parks department spends over 100 hours of labour pruning trees deemed “at risk” for developing Black Knot. Over the past few years, all of the seriously infected trees across Devon have been removed and replaced with a different tree with a much lower risk of being infected by Black Knot.

What can I do to help control 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
• Dispose of infected branches immediately. Once removed, diseased branches can continue to produce and release spores for months
• Educate yourself and your neighbours about black knot and how to properly manage it on your property

How to dispose of infected branches and cuttings

• Infected branches and other tree trimmings must be placed inside of a plastic bag and either burned or taken to the Town Yard Disposal Pile at 10 Exploration Drive
• Do not mix infected cuttings with other organic materials, including those in your green organics cart

Questions

Contact Devon's Parks department with any questions regarding Black Knot at 780-987-8316.