About Black Knot Disease

A fungus (Apiosporina morbosa ) causes Black knot disease, which results in black bulges on the branches of cherry and plum trees. The disease is native to North America. Overuse of chokecherries in urban tree planting projects has allowed the fungus to spread unnaturally fast. In Winnipeg, the disease is most often seen on Schubert chokecherries and can also attack plum and native chokecherry. Amur cherry and sour cherries are somewhat resistant to fungus.

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Healthy branch of Schubert chokecherry (left); early stage of infection (centre); later stages of infection (right).

What to Look For

The early signs of infection are hard to detect, beginning with brown nodules forming on the current or last year’s growth. The nodule will grow larger during the following year, turn green, and feel velvety to the touch. In the late stage of infection, the nodules are hard, black, and resemble burnt rope. Galls can be pruned out, and the sooner, the better. The earlier the galls are pruned, the less likely the fungus has spread further down the branch. Once the fungus reaches the trunk, the tree will rapidly decline. At this point, the conducting tissues in the trunk will become inhibited by the fungus and the movement of water and nutrients to the leaves will be restricted, ultimately causing the tree to die. 

Prevention and Treatment

Pruning out the infection galls will prolong the life of your tree, especially if done correctly. One of the essential parts of this practice is sterilizing your pruning tools with alcohol or methyl hydrate after each cut. Be sure to make proper cuts at the nodes when pruning out the knots. It is important to remove at least an extra 10 cm (4”) of healthy wood below the knot to make sure you prune out the entire infection. The recommended time to prune out black knot is late winter when new swellings are present. Cracking bark is another sign of the fungus, and you should remove all branches with these signs. Discard the branches in yard waste pickup.

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