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Alberta Agriculture and Forestry – Very few people think of pruning in the dead of winter, but it’s probably the best time when it comes to trees infested with black knot and fire blight.

“The beauty of winter pruning is that there are no leaves, and you will have a clear picture where to prune,” said Toso Bozic, a provincial woodlot extension specialist.

You want to remove the ‘three Ds’ — dead, diseased, and damaged branches, he said.

“Walking around your trees to envision the end result of your pruning is the first step,” said Bozic. “Next you can get to work removing water sprouts, suckers, rubbing and undesirable branches.”

Trees infested with black knot or fire blight require special care and attention.

“Black knot is a fungal disease while fire blight is bacterial, and you want to make sure you take the proper steps not to spread either of them,” said Bozic.

Black knot reduces growth, but takes a long time to kill a tree. On the other hand, fire blight is a very deadly disease that can kill part or sometimes an entire tree very quickly.

Black knot is a very common disease among the Prunus genus, and appears on a range of trees and shrubs including chokecherries, maydays, plums, and many others.

It is relatively easy to recognize due to the black and tar-like swelling of fruiting bodies. In the early stages of an infestation you will notice a green to olive-coloured gall (swelling) with a spur where the disease is spreading. After two to three years that fruiting body is mature and will ripen and become black. This stage is more recognizable.

Fire blight can kill a range of many hardy ornamentals or fruit-bearing trees and shrubs in the Rose family including crabapples, hawthorns, raspberry, saskatoon berry, mountain ash, pear, and cotoneasters.

In the summer, this disease is more easily recognizable as it causes blossom wilting and browning, and also causes leaves to turn black, as if they were scorched by fire. On the bark of branches you will see cracks, splitting, peeling, and eventually watery oozing. Infection will progress along the branch, circling it and creating a canker. The old infested canker may look like cracked bark with a black colour surrounding it. This is a good indication about where the disease is, and where to prune. These old cankers are the main source for a future fire blight infestation and must be removed.

“Pruning in cold weather is ideal for treating both of these diseases, as their spores are inactive and can’t survive cold temperatures,” said Bozic. “Winter pruning also removes the opportunity for them to spread and infest other trees.”

For both infestations, the key is to prune at least 12 inches below the infested area.

If an entire tree or shrub is infested, you will likely need to remove it. Once removed, infested branches must be destroyed immediately. You can put them in sealed bags and send them to the local landfill. Burning diseased branches is also an option in the winter. Do not leave the pruned branches on the ground until spring, as they can spread spores once the growing season begins.

You should still sterilize your pruning tools after each cut, but even if you cut through the spores or bacteria, they will be exposed to cold and can’t survive on tools for long in an Alberta winter.

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