Trees and plants are part of a larger ecosystem, which means they live alongside many insects, animals, birds, and other organisms. Most of those lifeforms are helpful, but some can negatively impact your trees. If you notice large black cankers that resemble charcoal on the twigs of your fruit trees, this is called black knot and is caused by a fungus. While many fungi are helpful, the one that causes black knot, called Apiosporina morbosa, can cause problems.
In this article, we will show you how to spot black knot, which trees it is most likely to impact, what happens if a tree has black knot, how to prevent or treat it, and more.
What trees are impacted by black knot?
Black knot is most commonly found on trees in the Prunus genus. Prunus trees include:
- Flowering almond
What does black knot look like?
Black knot first appears as a slight swelling in some of the branches of your fruit trees, usually visible in the fall. By the following year, the swellings enlarge and form into black galls that look similar to charcoal (thus the name “black knot”).
If the tree is particularly susceptible to black knot, the branch beyond the black gall will die and no new leaves will appear on that section. Some trees are more tolerant to black knot and leaves will still appear, but you may notice that the branches bend to one side.
You are most likely to notice black knot on your trees during the fall and winter seasons when leaves have fallen from your Prunus trees. In early summer the galls may be covered with green spores, but by late summer the entire gall will become black and hard.
A tree can have just one gall or hundreds of galls depending on the severity of the infection.
Sometimes black knot symptoms appear on a tree’s trunk in addition to the branches. Look for large areas of black bark. These areas may be swollen, cracked, or oozing a sticky liquid.
In some instances, cracks in the bark that are caused by this fungus can lead to other types of fungi entering through the cracks and leading to wood rot.
How does a tree get this fungus?
The Apiosporina morbosa fungus is spread in the spring when spores are released during wet periods and carried by the wind. Young saplings (trees under 3 years old) or trees with open wounds are most likely to become infected by these spores.
The galls that are formed, in turn, create their own spores to be released each spring.
What should I do if my tree has black knot?
Schedule an Inspection
If you spot what looks like black knot on any of your Prunus trees, contact Rayzor’s Edge for a tree inspection. If we confirm the diagnosis, our plant healthcare technicians can advise you on the best next step.
Prune out Infected Branches
When black knot is spotted on your trees, the next step is often to hire the Rayzor’s Edge team to carefully prune out any infected branches. Ideally, this pruning is done during the dormant season to prevent any further spread of spores.
Dispose of the Infected Branches
Infected branches should be properly removed from the area and disposed of to prevent the galls from releasing spores. This may involve burning the twigs (if and where allowed).
Most infected trees should be inspected (and possibly pruned) each winter.
Remove any Rotted Trees
If black knot has caused problems for your trunk, the professionals from Rayzor’s Edge Tree Service will need to determine if the tree is rotting. Rotting trees are weak trees and can cause significant damage if not removed.
How can I prevent black knot?
Before planting any Prunus trees, select a variety that is more resistant to black knot and the fungus that causes it. We recommend not planting any purple plum trees, for instance.
Japanese plum varieties (Prunus salicina) appear to have more resistance to black knot than some other varieties. The European plum (Prunus domestica) is also considered more resistant to black knot, especially the ‘President’ cultivar. For cherry trees, tart cherry varieties are often less susceptible.
If you already have infected Prunus trees on your property, avoid planting any other Prunus trees nearby. Prune out and destroy any infected branches before planting any similar trees anywhere on your property. Black knot is commonly found on wild cherry trees that naturally grow in our area, so check if your property has any that may be impacted.
Inspect all trees for evidence of black knot before planting them. We recommend not planting trees that show clear signs of black knot.
Not sure if your trees are impacted? Schedule an inspection
We realize that many property owners don’t have the time or the inclination to check for problems such as black knot on their trees.
Contact us today to schedule your assessment.
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