What are these black growths on my tree?

If you are wondering “why does my tree have these nasty black growths” and you live in or around Edmonton, you are probably experiencing black knot and you are not alone. This disease attacks Mayday and Schubert’s chokecherry trees. The black growths are the fungus taking hold of a branch. Each black knot growth on a tree can release millions of tiny spores that float through the air across the city and land on other trees. However the spores will only develop on the Mayday and Schubert’s cherry species. The disease is so prolific and widely spreading that there are spores all over town.

So what can be done?
Prune black knot from your tree as soon as you can. Millions of spores are introduced to your Mayday tree or Schubert’s chokecherry tree from air currents every day. Chokecherry trees seem to continue getting the disease even when you don’t see the infections in your neighbourhood. Stay on top of it early on. The first places the disease usually attack are the smaller branch tips. The tree can handle branches being pruned every year and will still grow. The problem you need to avoid is when the infection takes hold of a major branch or trunk. To remove the disease at this point that trunk needs to be cut off below the infection. This often results in a major loss to the trees canopy. Try to prune the tree at the earliest signs of black knot to avoid a complete removal.
Another solution is to replace the tree with a different species that is not a host to this pathogen- avoid the Mayday and Schubert’s chokecherry trees all together and you are safe from black knot- the disease is not hosted on other trees.

Winter and spring is the best time for you to hire an arborist to prune this disease because it is easier to spot the black knot growths on a leafless tree.

Mitigation strategies include pruning and regular monitoring for new infections.
In more severe infections the entire tree is removed. Diseased wood should be destroyed and removed. If you think you have black knot and would like a certified arborist to come out and assess it, fill out this contact form. Assessments are free.