August 15th, 2016
If you take a close look at cherry, plum and apricot trees here in Alberta, you may notice black, lumpy, burned looking masses growing on the tree branches. These unsightly bumps are likely caused by Black Knot, a fungal infection that is wreaking havoc on trees in the Prunus genus. The fungus works by causing the trees to turn against themselves; rapid growth of plant cells occurs until they distort and stunt the tree. If not removed, the knots caused by the overproduction of cells will spread lengthwise every year, slowly killing the tree. With some knowledge and mindfulness, we can help protect our trees from this invader.
In its early stages, Black Knot (also called black knuckle, or black wood) is insidious and can be easy to miss. The fungus spreads fastest during wet and warm springs. Keep an eye out for brown swelling on your trees' branches – this is the first sign that your tree may be infected. The following spring, the brown swelling changes to an olive green knot and as the tree opens its leaves, the fungus goes to spore, infecting other susceptible trees in the surrounding area. The more rain and wind we get, the easier the fungus spreads. The fungus eventually takes on distinctive black, burned and lumpy look.
Best care practices
Mayday and Schubert Chokecherries are the trees most susceptible to Black Knot here in Alberta, so keep an extra close watch on these trees. The only reliable method to deal with Black Knot is by pruning out the infected growth. This can be a tricky and technical task – our experienced arborists are here to help to ensure the proper steps are taken to protect your tree during pruning. If the Black Knot is severe, the best option may be to remove the tree. The fungus loves moisture, so try to avoid over-watering your lawn, and have your trees pruned regularly to promote proper air flow - keeping your branches drier and less welcoming to the fungus. If you notice Black Knot on your neighbour's trees, share your knowledge with them – they may not know the seriousness of the infection.
- The Latin name of the fungus is Apiosporina morbosa.
- The fungus was first described in Pennsylvania in 1821.
- Some trees are less susceptible to Black Knot, such as Nanking Cherry and Amur Chokecherry.