Gnarly fungus taking over Edmonton’s ornamental tree population

EDMONTON- Thousands of ornamental trees in Edmonton are being taken over by a fungus that causes black, gnarly knots to grow on them.

Black knot is a fungal disease which, in its early stages, appears as small light brown or green knots. Eventually, the knots become swollen, black and hard, and look burnt or charred.

The fungus suffocates and deforms branches, reducing their growth. Black knot thrives in warm, moist environments and has become more common in Edmonton over the last decade.

“When we have warmer, wetter springs like we’ve had in the past couple of years, it tends to get worse,” said Rob Sproule from Salisbury Greenhouse in Sherwood Park.

Cherry, apricot and plum trees are among those susceptible to the disease however, there are three species that are found in Edmonton that are particularly susceptible to black knot.

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“It’s your maydays, your pincherries, chokecherries. Those are the three main ones that we have here that have really become infected with this fungus,” explained Jeannette Wheeler, the principal of forestry for the City of Edmonton.

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“It’s transmitted by spores from the fungus,” Wheeler explained. “The wind, the insects, the birds, they can actually transfer this fungus from one tree to the next.”

The more susceptible tree species make up about four per cent of Edmonton’s urban forest population. Wheeler says the city can’t simply stop planting them, because it’s important that an urban forest be diverse.

“There isn’t the perfect tree that’s not susceptible to everything,” she said. “A diverse forest means that it’s quite stable so that there isn’t one pest or one disease that can come in a wipe out everything.”

Trees heavily infected by black knot often become stunted and may eventually die from the disease. However, diseased trees can live longer if affected areas are pruned.

Wheeler recommends pruning be done in late fall or winter, because the disease is dormant during those times.

“The reason we prune it in the winter time is one, we’re not releasing (the fungus) as we’re pruning (the trees), and also it’s really easy to see because it’s not being covered up by the vegetation or the leaves.”

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“The way to remove black knot is to prune the limb about six to eight inches down from the last bit of the infection. You need that buffer zone just in case there’s a little residual,” added Sproule.

While it isn’t visually appealing, black knot is not as serious as some other diseases such as dutch elm, because the number of trees that could become infected with black knot is much smaller.

For more information on black knot and how to properly prune infected trees, visit the City of Edmonton’s website.
With files from Shannon Greer.   



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