June 14, 2007 at 9:02 a.m.
Pruning out galls and cankers
Pruning out galls and cankers
Michelle Grabowski, U of M Regional Extension Educator, spoke at our last class on tree diseases. She is a plant pathologist who focuses on common diseases of plants in the yard and garden. Grabowski was most gracious when I asked if I could use part of her information for this article.
A wide variety of fungi and some bacteria can cause cankers and galls on Minnesota's trees and woody landscape plants.
A canker is an open wound on a branch or trunk of a tree where the bark and cambium have been killed. The bark is typically pulled back from the wound or sunken in. Sap, resin or fungal fruiting bodies maybe present at the site of the canker.
Both cankers and galls interrupt the natural flow of water and nutrients through a branch or stem. As the canker or gall grows, it can completely girdle and kill the branch. Fortunately for gardeners, many cankers and galls start on small branches and can easily be pruned out and destroyed.
The many fungi and bacteria that cause cankers and galls have a different capability in hurting a tree. Some galls seem only to slow the tree down, where other galls like black knot can kill many branches on an infected tree. Trees can heal over some cankers, but others spread rapidly, killing branches and sometimes the whole tree. Some common cankers and galls in Minnesota include Black Knot, Cytospora Canker, and Fireblight.
Black Knot is fungal disease that infects edible and ornamental species of wild plum, black cherry, and pin cherry. Black knot galls are rough, black, cracked and swollen area along the branch. In the spring black galls shoot spores into the air when it rains. Infection is at its peak when young shoots are growing and temperatures are between 70 degrees and 75 degrees F.
To remove the galls, cut the branch at least four inches below the swelling to ensure that all the fungus is removed form the tree. The pruned galls must be removed from the area and destroyed because the spores can be produced on the pruned off branches.
Fire blight is another common disease that requires dormant season pruning. This is a bacterial disease that infects apple trees, roses, raspberries, service berries, and mountain ash. The bacteria infect plants through the wound and natural openings. As the infection spreads, flowers, leaves, shoots, and branches turn black and die. The infection causes cankers on branches that look like sunken, cracked dark areas. These cankers ooze millions of watery bacteria. To prevent spread you should only prune during the dormant season. The cankers should be cut out six inches below the sunken dark area, and destroyed. All tools should be disinfected with isopropyl alcohol, 10% bleach solution, or a disinfectant like Lysol.
Cytospora canker is a fungal disease that can infect several kinds of evergreen trees but is mostly seen on spruce trees. These cankers are sunken and completely covered with resin or sap. It may take years but the canker will eventually girdle and kill the branch. They release spores in wet weather all year long, but mostly in the spring. Although cankers that have not yet killed a branch can be left on the tree, dead branches should be pruned out during February or March.
Although you should not prune out cankers and galls during the growing season, this is an excellent time to search and identify these problems. Then you will be prepared to remove and destroy them during February and March.
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Our spring series has ended, but the Monday night plant clinics have begun! Mondays, from 4:30-7 p.m., volunteer Master Gardeners are available to answer your garden questions. You can stop by the office at 38780 8th Avenue, in North Branch, or you can call the Extension office at 651-674-4417 to speak with a Master Gardener.
If you visit our website at www.extension.umn.edu/county/chisago you will find instructions for handling samples.
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