Black knot fungus infecting thousands of Calgary trees

An air-borne fungus called black knot is infecting thousands of trees in Calgary, and arborists say they are struggling to keep up.

Arborists say they are struggling to keep up

Black knot has infected this tree in the southwest Calgary community of Cougar Ridge. (Karen Moxley/CBC)

An air-borne fungus is infecting thousands of trees in Calgary. The disease is called black knot, and it infects trees in the prunus family. 

Schubert Chokecherry and Mayday trees in the city are particularly affected by the fungus, which looks like black clumps of coal that wrap around twigs, branches and even the trunks of trees in some extreme cases.

Black knot has been keeping arborists in the city very busy this spring. 

"I'd say a good 40 per cent of everything we've done in the last eight months was black knot related," said Matthew Davis, an arborist with Adair Tree Care in Calgary. 

"Because Schubert Chokecherry and Mayday trees are so commonly planted, it's hard to go to far without finding a yard that has one of these trees in it."

Fungus thrives on moisture

Black knot has been in Calgary for decades, but experts say the last couple of soggy springs has made the disease worse this year.

We've lost at least 50 trees in Discovery Ridge.- Ward 6 Coun. Richard Pootmans

Black knot is infecting trees in every quadrant of the city, but newer communities in the southwest are particularly bad. 

Trees on private property, as well as city land are affected. 

The city is very aware of the problem, and crews are trying to keep up with 311 calls. 

"We've lost at least 50 trees in Discovery Ridge," said Ward 6 Coun. Richard Pootmans.

Discovery Ridge hit bad this year


The most distinguishing symptom of black knot is the characteristic black, tar-like swellings that develop on branches of the infected plant. 

It can stop nutrients from passing through, and eventually it kills the branches.

Although the fungal disease is rarely fatal for the entire tree, if left unchecked it will reduce the vigour and ornamental value.

Pootmans said black knot was so bad in Discovery Ridge that the city worked out a cost-sharing agreement with the community association to remove and replant the infected trees along the neighbourhood's main boulevard.

"Residents find it heartbreaking," said Pootmans. 

"We're going through other communities in West Springs and Cougar Ridge, looking at similar problems, and dealing with this as quickly and as cleanly as we can." 

Lars Lehman, the president of the West Springs Cougar Ridge Community Association, says he has seen the problem in his own backyard. 

"I had a flowering tree in my backyard that had the issue. And I'm noticing black knot on other trees in the neighbourhood as well," said Lehman. 

Schubert Chokecherry, Mayday trees affected

"It's important that everybody goes into their own yards and takes a look and prunes. Because if I get rid of black knot on my tree, that helps the people downwind from me, but if there are people upwind from me that haven't taken care of it, there's a good possibility I'll end up with the same problem again later this year," Lehman added.

A couple of years ago, both developers and the city went through a phase of planting many Schubert Chokecherry and Mayday trees in newly-developed neighbourhoods, despite knowing the trees were susceptible to black knot. 

"These trees do provide an enhancement to the aesthetics of the community," said James Borrow, the pest management lead for the City of Calgary. 

Schubert Chokecherry trees have red leaves through the summer months, and Mayday trees produce fragrant clusters of white blossoms. 

"We're pretty limited by the number to trees that survive in the chinook zone, so that's partly why these trees were planted," said Borrow.

Tips for removing black knot

"Even though [black knot] is endemic in the environment, it won't necessarily cause the death of the tree if the trees are managed properly to remove the infected branches."

To mitigate the affects of black knot, residents are urged to monitor trees on their own property. 

If black knot is found, experts advise to remove the branches six to eight inches below the affected area. And always disinfect pruning tools between cuts, to avoid spreading the disease. 

Infected branches should either buried, burned or disposed of in a landfill. 

Click here for more information from the City of Calgary about black knot, and how to remove the fungus. The video below is a tutorial on how to deal with black knot. On mobile? Click here.


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