Black Knot Disease
Black knot is a tree and shrub disease that affects members of the Prunus family like chokecherry and pin cherry, and is a common problem for residents of Wood Buffalo. You may have seen it in your yard or neighbourhood.
You can easily identify the disease. It develops on twigs and branches, and looks like lumps of thick, black, tar-like swelling that may vary in size from 1/2 inch to more than 12 inches in length.
Black knot has been reported on 24 species of Prunus in North America, but is most commonly found on wild and cultivated plum and cherry trees.
The Problem with Black Knot Disease
Black knot disease damages and kills trees by affecting their branches. Black Knot will dwarf and misshape trees, making them stressed and unhealthy, and can infect other healthy trees in your yard, your neighbour’s yard, and throughout your neighbourhood.
If not controlled, the disease destroys garden aesthetics, disfigures branches, and eventually kills trees.
Young, infected twigs may die during the first year of infection. Larger branches may take several years to show signs of damage. The infected trees decline and become more symptomatic with each growing season. The infection stresses the entire tree, causing it to weaken, decline and eventually die.
How to eliminate Black Knot Disease
Fortunately, this fungus is easy to remove by cutting off diseased branches and disposing of them immediately. We recommend taking them to the landfill. Do not compost or leave them in your yard.
It’s easier to identify infected branches and twigs in the winter. It’s also the best time to remove any suspicious branches and twigs as the spores will not spread as readily, protecting your trees and other trees in the area.
Pruning Tips and Techniques
- All shoots and branches bearing knots should be pruned during the winter. This pruning should be completed before the time that the buds first break in the spring.
- When pruning, cuts should be made at least 15-20 cm (6-8 in.) below the knot.
- Use sharp pruning tools like clippers, shears or saws to make clean cuts.
- Sterilize your tools between cuts by dipping them in a 1:3 bleach and water solution, or by using a dish soap solution.
- Dispose of diseased branches by putting them in the trash, burying or burning them. Do not put them in the compost or keep them in your yard, as they can spread spores for months after removal.
Spraying the tree with a fungicide will help prevent new infections but will not cure disease already present. A copper fungicide should be applied every two weeks from the time the leaf buds open until three weeks after flower petals fall. There are many formulations of copper fungicides and the fungicide you use must be labeled specifically for black knot fungus.
Replacing Your Trees
Sometimes the disease will have progressed enough that the tree cannot be saved and removal is necessary. You can always plant another tree once the diseased tree has been properly removed. When selecting trees for a new planting, consider varieties that will do well in our climate, like the Amur chokecherry (Prunus maackii).
Contact us for more information on how to eliminate black knot disease from your property.
Parks Services: 780-799-5832
After hours: Pulse Call Line: 780-743-7000 (evenings, weekends and holidays)