Black Knot Disease

Black knot disease is caused by a fungus (Apiosporina morbosa ) that results in black bulges on the branches of cherry and plum trees. The disease is native to North America, but the 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 other cherry trees. Amur cherry and sour cherries are somewhat resistant to the fungus.

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 is hard to detect, beginning with brown nodules forming on the current or last year’s growth. During the next year, the nodule will grow larger, turn green, and will 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 out, the less likely that 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 most important parts of this practice is to sterilize your pruning tools with bleach, alcohol, or methyl hydrate after each cut. Be sure to make proper cuts at the nodes when pruning out the knots, as described on our Pruning page. It is important to remove at least an extra 10cm (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 all branches with such signs should also be removed. Discard the branches in yard waste pickup or burn the branches to prevent the knots from releasing airborne spores which can re-infect the tree.