Black Knot

Black knot is a fungal disease that has become an increasing problem in Alberta. Affecting primarily Mayday and Schubert Chokecherry trees, it can spread to Plum, Apricot and Cherry trees. Once spotted, Black knot should be removed to help control the fungus from spreading within the canopy and to healthy trees around.

What does Black knot look like?

At first, it may be very difficult to spot Black knot. Greenish brown areas will swell and are hard to spot with the un-trained eye. Eventually, the areas darken and harden to form a black, tar like deformation. It is easier to spot when the tree is in it’s dormant state, as the leaves are off the tree. Black knot can be obvious, present in many affected branches of the tree, or present as a single affected swelling.

How does it spread?

Like all fungus, Black knot spores thrive with periods of warm, humid weather. Wind, insects and wildlife can also help with transport. Black knot will spread within the canopy of the tree as well as to other trees around. Black knot will even spread into the main stem of a tree.

I think my tree has Black knot. What should I do?

Consider hiring a professional to properly prune your tree. It is usually best to remove Black knot when the disease is in its dormant state, leaves are off the tree, and it’s much easier to see. If you do plan on removing Black knot yourself, plan to sterilize your equipment during use, and prune back to at least 6-8 inches from the visible knot. Sadly, sometimes trees are so badly ridden with Black knot they have to be removed. An ISA trained Arborist will be able to give you guidance on whether the fungus is maintainable or if the tree needs to go.