Edmonton Trees and the Adverse Effects of Black Knot Disease

What is Black Knot Disease?

Black Knot Disease is essentially fungus on tree branches, or as we like to refer to it at OutdoorSpace as “Turds on a Stick” is caused by the fungus Apiosporina morbosa and is a very common disease of plants in the genus Prunus category.

Have you seen trees (see image below) that look like this in your yard or your neighbor’s yard? The things hanging from the trees are NOT Christmas ornaments, they are “Black Knots” and… they MUST to be dealt with!

Black Knot Disease

Edmonton Tree Species Specifically Affected by this Disease

  • Amur Cherry
  • Mayday Tree
  • Apricot
  • Mongolian Cherry
  • Black Cherry
  • Nanking Cherry
  • Chokecherry
  • Pin Cherry
  • Dropmore Cherry
  • Cultivated Plum
  • Flowering Almond
  • Wild Plum
  • Flowering Plum
  • Prunus Hybrids
  • Japanese Plum
  • Sand Cherry
  • Korean Cherry
  • Sour Cherry

How did my tree get affected? This is how the Black Knot cycle works…

Black Knot Disease

How to identify the disease… the disease initially shows its face as small little (speckled) spores that are “orange/brownish” in colour and look like the spots found on Trout. The first signs of (new) infection are visible by early Fall. Only succulent green twigs of the current season’s growth are susceptible to infection. Smaller twigs typically die within a year. Several years may pass before larger branches are girdled (encompassed) and die from the fungus. Over time the infected areas will turn black and start to swell. The swelling will grow until it is mature after approximately 2-3 years. The mature galls are hard, black, 10 to 15 cm (4 to 6 inches) and may be somewhat ruptured (split). Mature galls will produce and release a vast number of spores during the blooming period, resulting in a rapid increase in infections throughout the affected tree. The fungus continues to grow both internally and externally, with the branch eventually becoming girdled (encompassed) and dying altogether.

Black Knot Disease

Fact is… if you do not stay on top of this disease… it WILL eventually KILL the tree altogether!

When is the BEST time to inspect my Trees for this disease?
The BEST time to inspect your Trees for this disease is in the Fall or Winter and BEFORE the tree starts to blossom in the Spring, as the disease is fully exposed and less likely to be camouflaged by the new leaves.

When is the BEST time to remove the disease?
The BEST time to remove the Black Knot Disease effected branches and prune your trees is when the tree is in its “dormant” state. This time frame extends from late Fall to early Spring and just before the leaves start to bud out. During this time the sap (essential nutrients/food) isn’t flowing, and as a result the tree is less likely to succumb to shock or disease from the open and fresh cut.

How do I remove the infected branches?
Remove infected branches to at least 15-20 cm (6-8 inches) below the knot. NOTE: It is preferable to prune an infected branch further back to an appropriate location, such as a healthy collar, rather than leave an unsightly looking stub. As a precaution, cutting blades should be cleaned and disinfected after pruning and especially if cuts have been made through infected areas. 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.


(burned, buried or removed from site).
Diseased knots can produce and release spores for up to 4 months after removal. Proper composting can also help to accelerate the breakdown of infected materials

Ongoing Maintenance…

  • Regular (annual) monitoring of your trees
  • Bring this issue to the attention of your neighbors if they have trees that are infected
  • Ensure adequate canopy ventilation with the incorporation of ongoing and proper pruning techniques
  • Chemical (fungicide) control can be implemented in order to protect young or highly susceptible trees from infection. Spay must be applied in early Spring in order to protect young green shoots. Please note: this method is considered as a “preventative” measure and not considered as a “cure” for branches and twigs that are already infected

We hope you found this article on Black Knot Disease to be both educational and informative!

Should you have any questions, we would love to hear from you!

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