A deadly tree fungus is spreading across Edmonton, and gardening experts say the sooner its dealt with, the better.

Rob Sproule, owner of Salisbury Greenhouse gardening centre, says the fungus is called "black knot" and it can kill a tree in just a few years if you don't take action.

The fungus causes irregular swelling on a tree's branches. At first, the swelling is olive green, like new wood, but it then turns black and hard, and begins strangling the affected branch.

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Black knot fungus spreads by spores and infects a tree's cells, causing an overgrowth that eventually kills the tree. (CBC)

Sproule said the infection is easy to spot.

"Just look out into your trees and look for the silhouette, the thickened black, it almost looks like charred black rope is wrapped around it or burnt sausage links, something like that," he said.

Sproule said catching the fungus early and pruning the infected branches can usually save the tree.

"If you get rid of all the knots, the tree should be fine," he said.

But because the fungus spreads through spores, he said not to compost the branches. Instead, wrap them in plastic and throw them away in the regular garbage.

Sproule also said one should disinfect or sanitize pruning shears between each cut in order to reduce the likelihood of spreading the spores to a new branch.

Black knot fungus targets mainly mayday and fruit trees, and has become a problem elsewhere in urban Canada in recent years.

In 2012, the City of Winnipeg took agressive steps to slow the spread of the fungus in some of its newer neighbourhoods, removing around 800 city-planted chokecherry trees that were infected.

With files from the CBC's Brandy Yanchyk