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Black knot

branches with black lumps

Black knot in chokecherry

M. Grabowski, UMN Extension

Black knot is a very common disease in landscape and native Prunus trees and shrubs, which occasionally infects susceptible plum, apricot and cherry varieties. It is caused by the fungus Apiosporina morbosa. Black knot causes branches and twigs to develop a raised, rough, tumor-like black growth called a gall. The black knot gall causes infected branches to either weaken or die. Left uncontrolled, the disease spreads to other branches on the same tree, until the tree becomes so weak that it no longer produces fruit and should be removed. In most cases, black knot can be controlled with careful pruning. During dormancy remove and destroy all knots, or all swollen stems. Cut infected areas four inches below the visible swelling. More information about the biology and management of black knot can be found in the UMN extension publication 'Black knot'.

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