Black knot is a fungal infection that certain deciduous trees are prone to. Trees in the prunus genus such as plum, cherry, and especially maydays and Schubert chokecherries are the trees to watch. Black knot looks like a charred, shiny, black bumpy mass that grows on the tree's branches and is easy to spot during the winter months when the trees are bare of leaves. Not only does it ruin the appearance of the tree, but if left unchecked it will eventually kill the tree.
Black knot causes rapid growth of plant cells until they distort and stunt the tree. In time as the masses grow longer they will engulf the entire branch, preventing it from taking in moisture and nutrients, slowly causing the tree to die.
By the time it is noticed it has already been on the tree for a couple of years. It starts off as a small olive green swelling that matures into the black knotty growth that will throw out large amounts of spores when it is active. It is active in the spring, just as the tree breaks into leaf and it peaks as it goes into bloom. It spreads even more quickly during warm, moist conditions.
The easiest and most effective way to control black knot is to prune off the affected branches. Ideally, the best time to do this, is in late winter/early spring when the fungus is dormant.
During the spring when temperatures warm up and the air is humid black knot becomes active and this is when the spores are spread, so when you are pruning it is very easy to spread the spores to other parts of the tree as well as surrounding trees, and also to the open cut.
When you prune off the infected branches, cut at least 10-15 cm down from the knot. When pruning keep in mind that you want to prune above a bud or in the case of the entire branch being removed, to the joint, leaving a healthy collar. If there is black knot on major branches that can't be removed then you will have to cut out the black knot until you reach healthy wood.
After each cut, sterilize the pruners with a bleach solution (one part bleach to three parts water) to help stop the spread of the fungus. It is very important to destroy or throw out the infected branches immediately as they can continue to throw out spores weeks after they have been pruned. They can be burnt or buried, or placed in a plastic bag and put in the garbage.
To try and prevent black knot, keep trees healthy and stress-free. Keep up on the fertilizer and water, preferably using a drip system so you only water the tree roots keeping the branches and foliage dry, as black knot thrives in moist conditions. Regular pruning and creating good air ventilation is helpful.
If you see black knot in your neighbours tree, alert them so that they can destroy it before it spreads to nearby trees. By being proactive and watching for any problems, and treating them quickly, you should be able to keep black knot under control.
