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Friday, March 2, 2018

Prune out cankers and galls now

Michelle Grabowski, UMN Extension Educator
Black knot gall on a wild cherry tree.
M. Grbaowski, UMN Extension

An important late winter gardening task is pruning to remove diseased branches. Winter is the best time to prune for several reasons. Diseased and damaged branches are easily seen when leaves are not present. Winter temperatures are not ideal for the fungi and bacteria that cause disease in trees and shrubs. The pathogens will still be present but at much lower levels than during the growing season. By pruning out infected branches now, gardeners can significantly reduce the risk of the disease spreading within the plant and to neighboring plants once the growing season begins.

Several types of disease can affect branches of landscape trees and shrubs.

Gall rust on a pine tree. M. Grabowski,
UMN Extension
A gall is an unusual overgrowth of plant tissue caused by a pest or pathogen. Fungi and some bacteria can cause galls to form on branches of trees and shrubs. Galls can be round, oblong, or irregular in shape. They are made of wood but may have discolored bark due to the infection. Galls can girdle a branch and kill all leaves and shoots beyond the gall. Some trees and shrubs will tolerate galls and will not suffer any damage as a result.

Golden canker causes the reddish purple
bark of pagoda dogwood to turn yellow.
M. Grbaowski, UMN Extension 
A canker is an infection of the sapwood and living bark caused by a fungal or bacterial plant pathogen. Cracked, discolored, or blistered bark in an isolated area of a branch indicate that there is a canker. In some trees and shrubs a thick hard layer of resin or sap may cover the infection. If the bark is peeled back, reddish brown discoloration of the wood can often be seen.

A canker will eventually grow to encircle the branch, cutting off the flow of water and nutrients. This results in death of the branch, and any shoots and leaves beyond the canker. In some diseases, the canker can progress into the main trunk and result in death of the tree.

Make the pruning cut a minimum of
4 inches below visible symptoms at
a branch union. M. Grabowski,
UMN Extension 
How to prune out cankers and galls
First examine the tree carefully to find any branches with discolored, cracked or blistered bark or with any unusual tumor like growths. The infection will extend beyond the visible discoloration of the bark so the pruning cut should be made at a minimum of 4 inches below symptoms of disease. The tree will heal the pruning cut most rapidly and easily if the cut is made just above a bud or a branch union.

All branches with galls or cankers should be removed from the area and burned or buried. Pruning tools should be cleaned with undiluted Lysol (active ingredient .1% alkyl dimethylbenzyl ammonium saccharinate) or a 10% solution of household bleach in water.

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