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Finalize pruning for tree health

Black Knot on Prunus.
M. Grabowski, UMN Extension

As a general rule of thumb gardeners are told to prune out any diseased branches from trees and shrubs before the end of March. The biology behind this rule tells us that during cold winter weather, trees and the microorganisms that cause tree diseases are dormant. When the weather warms up, trees become active and so do their pathogens. Pruning cuts made in cold weather are less likely to become infected with the pathogen being pruned out or any other pathogen.

Fire blight canker on crabapple

In addition, many plant pathogens of trees and shrubs overwinter in infected branches. Common examples are black knot galls on Prunus spp. caused by the fungus Apiosporina morbosa,  fire blight cankers on crabapples, apples and mountain ash tress caused by the bacteria Erwinia amylovora and golden canker of pagoda dogwood.

Golden Canker on Pagoda Dogwood
Cankers and galls should be pruned out by making a pruning cut 6-8 inches below visible symptoms of the disease (cracked, swollen or discolored bark are common symptoms). This will ensure that all of the pathogen is removed from the tree and only healthy tissue remains. Infected branches can be burned, buried or brought to a municipal or commercial composting facility.
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