Channel your spring fever to tackle the nasty 'black knot' fungus infecting Edmonton trees

·3 min read
A closeup of the black knot fungus, which impacts many fruit and mayday trees in Edmonton.  (Tricia Simon/Alberta.ca - image credit)
A closeup of the black knot fungus, which impacts many fruit and mayday trees in Edmonton. (Tricia Simon/Alberta.ca - image credit)

Next time you're out in the yard, you might want to check to see if you have what looks like burnt sausages hanging off your tree branch.

The ropey black growths are the sign of a disease called black knot fungus. If they're not properly removed, the fungus will kill the tree and spread to others, says Rob Sproule, co-owner of Salisbury Greenhouse in Sherwood Park.

"Black knot fungus is kind of a nasty critter that's been around for a long time. It's hard to get rid of, but easy to spot," Sproule told CBC Radio's Edmonton AM this week.

"Now is a good time to look for it because there's no leaves on the tree and you have an unobstructed view. It looks like burnt rope or burnt sausages on the branches. The branches have actually turned into, you know, the burnt sausage."

The fungus tends to attack trees of the prunus genus, which includes fruit trees like plum and cherry as well as the flowering mayday trees that are popular in the Edmonton area.

Watch | What is black knot fungus?

Both Sproule and the city's forestry experts say that right now — before the leaves start emerging — is the time to tackle the problem.

"Black knot is relatively common in Edmonton," according to Melissa Campbell, the city's supervisor of urban forestry operations.

"Each year, city crews prune disease-prone trees during the winter months when the tree is dormant."

During the most recent pruning season, which runs from October through March, crews dealt with more than 2,600 trees that had, or are susceptible to, black knot. Another 390 infested trees were removed, Campbell said.

Another good reason to tackle the problem now, Spoule said, is that fungus thrives in wet weather.

A tree infested with black knot fungus. If not dealt with, the fungus will move into the stem of the tree and eventually kill it.
A tree infested with black knot fungus. If not dealt with, the fungus will move into the stem of the tree and eventually kill it.(Tricia Simon/Alberta.ca)

"It spores when it starts to rain. So the more it rains in the spring, the more it's going to spore out," he said.

"You actually start to see a white film on it, that's the spores. And when the wind picks up, it carries them through the yard, into the neighbour's [yard] and you're not very popular."

The disease initially will show up like small, light brown swelling, usually on new green stems. Over time, the knots will darken and harden.

Pruning involves getting rid of infected branches, about 10 to 15 centimetres past the knot. The city notes that it's better to prune an infected branch further back than to leave a stub.

Experts also advise wiping the pruning tools with bleach between cuts to avoid spreading the spores.

Equally important is properly disposing of the infected branches, he said.

"It's pretty toxic waste. It can actually continue to spore out for four years afterwards. … It's nasty business," Sproule said.

"You want to either burn it, if you're in a rural area, or wrap it in a sealed plastic bag and put it in the garbage that way. It definitely doesn't just go into the compost."

Sproule said black knot fungus has been around for hundreds of years but, because of the types of trees it affects, it's more likely to be found among trees planted within the city than in natural areas.

"It's not going away, but we just have to learn how to control it," he said.

"For a lot of people who really have the spring fever — spring fever is huge this year — it's a great time to do pruning and get out in the yard and actually do something."