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  • Writer's picturePeter LaRue

Black Knot Fungus

Welcome Back to the LaRueTree Certified Arborists Blog! This time, I've decided to write about a fungal infection of trees in Edmonton, Vegreville, and all of the prairie provinces. This disease is not an introduced species, but a native fungus that has taken a stronghold on our urban and native tree population.

Driving around any prairie town or city, you might notice clumps of dark tissue on the branches and trunks of our ornamental cherry species; Mayday Cherry (Prunus padus) and Schubert Chokecherry (Prunus virginiana 'Schubert'), and our native Cherry species; Pin Cherry (Prunus pensylvanica). These black swellings are the telltale sign of the black knot fungus (Apiosporina Morbosa). As an Arborist I always have my eyes on the trees, and this disease is one of the most visible.


Why should you be concerned about this disease?

First and foremost, Black knot is an aesthetic concern. It is extremely unsitely in an urban setting, turning ornamental features into sick looking specimens. Black knot is most visible in the winter, as the leaves do a great job of hiding the early stages of this infection. As the fruiting bodies start to appear, the diseased tree will show the effects of the disease. Branches will die back, and the knots will multiply in number. Trees can survive for a number of years with the disease, but it is likely that they will succumb to the ill effects that it has on their overall health.

When is the best time to prune out Black knot?

Black knot fungus spores out in warm, wet weather, typically around 20° C and rain. So, the best time to prune out this disease is during it's dormancy, in the late fall and winter. Winter is also the easiest time of year to spot the early swellings on the young twigs and branches before the disease has a chance to spore out and spread to surrounding trees. As the knots spore out, their billions of spores seek to infect any open wound, succulent growing point or fruit spur on their host tree or surrounding trees, so timely removal is imperative in slowing down this disease. After removal of the diseased tissue, it is extremely important to dispose of the infections immediately. Spores can remain active for up to 4 months even after removed from the tree.

For a more scientific approach, read through the Alberta Governments Black Knot page:

How Can LaRueTree Help?

Proper diagnosis and treatment is important if the health of your urban forest is to be maintained. Once your yard has been assessed, we can provide professional pruning and removal services. Our estimates for services are always free, and we are more than willing to share our knowledge with you, there's nothing I like more than talking about trees!

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