Check bare branches for cankers

When the branches of your trees and shrubs are bare in winter, you have an opportunity to spot cankers that might be hidden by leaves in summer.

A canker is an area of disease in the wood of a plant. “We can’t cure cankers,” said Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum in Lisle. “We can only cut them out.”

Winter is a good time to prune out cankers since disease pathogens are less likely to spread in freezing weather while plants are dormant.

“Cankers may involve a change of color of the stem, a sunken area, cracked or peeling bark or oozing sap,” Yiesla said. The affected branch may die back from the tip down to the canker and beyond.

Cankers can be caused by a variety of pathogens, although most are caused by fungi, she said. Some kinds of canker diseases are specific to certain host plants, but others can affect any tree or shrub.

Cankers usually start at a wound that gives them a way into the plant. It may be a pruning wound, a broken branch, a scrape or nick in a young tree’s bark from careless handling during planting, or a gouge in the bark from a lawn mower or string trimmer.

If a tree is already weakened — for example, by drought because it hasn’t been watered in the heat of summer — a canker disease may find another way in. “That’s one reason it’s important to keep your plants in good health,” Yiesla said. “Healthy plants have better defenses against disease.”

Since cankers can have such a wide range of appearances, they can be a challenge to identify. “To look for a canker, find a stem that is showing dieback, oozing, darkened bark, or other signs of distress. Follow the stem back toward the base until you find the wound where the canker pathogens got in,” she said.

If you are unsure whether the damage you see is a canker, take photos and use them to consult the Plant Clinic for help in identification (mortonarb.org/plant-clinic).

To remove a canker, first disinfect your pruners with 70% rubbing alcohol, sold at drugstores. You can dip the pruners into a container of the alcohol, making sure the whole blades are submerged, or spray the blades with alcohol using a small spray bottle. “It’s critical to work with clean, disinfected tools so you don’t spread the disease even more,” Yiesla said.

Then cut the branch off at least 6 inches below the canker. “The cankered area you can see may not be the end of it, since the disease can spread under the bark undetected,” she said. If you see a dark color under the bark when you cut, cut further down the stem until you reach wood that shows no discoloration.

Disinfect your pruners after every cut. After you have removed the diseased wood and cut it up for disposal, disinfect your pruners one last time to make sure you don’t spread the disease to another plant.

To prevent cankers, keep trees and shrubs healthy by watering them when it’s dry. Avoid causing wounds in the bark of trees or shrubs. Be very careful not to nick their bark with power tools. Use sharp, disinfected pruners whenever you prune. Don’t tie a hammock or a swing to a tree’s trunk or branches, because the rubbing of the rope can cause wounds in the bark.

Take special care when planting young trees, which have especially tender bark, to avoid damaging them. “Never lift a young tree by the trunk when you’re planting and be careful not to hit it with your tools,” Yiesla said. “Any wound can be an open door to disease.”

For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or [email protected]). Beth Botts is a staff writer at the Arboretum.

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