Black Knot Fungus: The Ugly Black Growths on Calgary Chokecherry and Mayday Trees

If you have a mayday, a chokecherry, or an ornamental plum anywhere in Calgary, you have almost certainly seen it: hard, black, lumpy growths wrapped around the branches like lengths of burnt charcoal or lumps of coal fused to the wood. That is black knot, and it is one of the most widespread tree diseases in this city. Once you learn to recognize it, you start seeing it in nearly every back lane.

Left alone, black knot spreads relentlessly, adding new galls every year, killing branches, and eventually disfiguring or dooming the whole tree. The frustrating part for most homeowners is that the disease is usually well established by the time anyone notices those first black lumps, because the early infection is easy to miss and the galls take a season or two to darken and swell.

What Black Knot Is Doing to Your Tree

Black knot is a fungal disease that infects the woody stems of trees in the Prunus family. Around Calgary that means chokecherry, mayday, plum, and especially the Schubert chokecherry, that purple-leafed favourite planted along boulevards and in front yards all over the city. Because so many of these trees are packed into our neighbourhoods, the disease has an abundance of hosts to jump between.

The fungus stimulates the infected branch to swell and produce that hard, rough, black gall you see wrapped around the wood. As the gall grows it strangles the branch, choking off the flow of water and nutrients past that point. That is why the portion of the branch beyond a knot so often wilts, browns, and dies, while the knot itself keeps expanding and maturing.

How It Spreads So Fast

The disease has a yearly cycle that makes it a real neighbourhood problem. In spring, particularly during wet, humid weather, the mature knots release clouds of microscopic spores. Wind and splashing rain carry those spores to the tender new growth on the same tree and on any susceptible trees nearby, where they infect fresh shoots that will not show visible knots until the following year.

This is exactly why black knot spreads so quickly across a block. If your tree is infected, the odds are good that spores are also landing on your neighbour's mayday and their chokecherry hedge, and theirs on yours. Trying to control it on one tree while ignoring a heavily infected tree next door is an uphill battle, which is why it often pays to talk to the neighbours too.

Pruning Is the Only Real Fix

There is no spray, drench, or injection that cures black knot once a branch is infected. Anyone promising a spray-on cure is overselling. The proven, time-tested approach is mechanical: cutting the knots out of the tree, removing the infected wood, and getting rid of it. Done properly and consistently, pruning genuinely controls the disease and can keep a valued tree healthy for years.

The key is cutting far enough back. Remove each knot along with at least 15 to 20 centimetres of apparently healthy branch below it, because the fungus grows inside the wood well beyond the visible black growth. A timid cut that just trims off the lump leaves the infection behind, and the knot simply reappears a little further down the same branch the next season.

Timing and Clean Tools

The best window to prune out black knot is late winter or very early spring, while the tree is fully dormant and the knots are not yet actively releasing spores. Pruning during warm, wet spring weather, when the galls are shedding spores, risks spreading the disease around the very tree you are trying to save, so timing your cuts is not a small detail.

Disinfect your pruning tools between cuts so you are not carrying fungal material from an infected branch straight into a fresh wound on a healthy one. Just as important, never leave the pruned knots lying around the yard, in a brush pile, or in the back lane. Detached knots can still mature and release spores from the ground, so bag them up and get them completely off the property, ideally to the landfill rather than the compost.

When to Save It and When to Remove It

A tree caught early, with only a handful of knots out on the outer branches, is usually very saveable, and consistent yearly pruning can keep it in good shape indefinitely. The prognosis is genuinely good when you get ahead of it. The picture changes when knots appear on the main trunk or on the large scaffold limbs, or when there are dozens of galls scattered throughout the crown.

At that stage the disease is often past the point where pruning can win, and cutting out that much wood would leave little tree behind. Because black knot readily spreads to nearby trees, a badly infected specimen is sometimes best removed entirely to protect the rest of your yard and your neighbours' trees. Aardvark Tree Care can assess how far it has progressed and give you a straight, honest answer on whether to prune or remove.

There is no spray that cures black knot; cutting it out well below the gall is the only real fix.