Tree problems · Disease
Black knot: signs and what to do
Black knot is a tree disease, often seen on black cherry, plums and other cherry trees. Look for: rough, swollen, tumor-like growths on twigs and branches.
General information, not a diagnosis. A certified arborist or your county extension office can confirm.
- Disease
- Black cherry
- Plums and other cherry trees
- Other Prunus (stone fruit) trees and shrubs
Signs to look for
Tick what you see. More ticks make it more likely, but only an on-site look can confirm.
0 of 6 signs ticked. Ticks stay in this browser only.
Season
Infection happens in spring on young growth during wet weather. Knots are barely visible the first season, then swell and turn black in later seasons. They are easiest to spot in winter.
How it spreads
Spores from mature knots are carried by wind and splashing rain to young shoots. Leaving old knots on the tree or nearby wild cherry and plum trees keeps the disease going.
Where it is found
Black knot is a common disease of cherry and plum trees across the United States, mainly where Prunus trees grow in humid areas. Check with your state extension office for local conditions.
Official sources
What to do
- 1
Look over cherry and plum trees in winter for black, rough growths on the twigs and branches.
- 2
Prune out the knots during the dormant season. Extension sources advise cutting several inches below the knot into healthy wood.
- 3
Take the cut pieces away from the tree and dispose of them as your extension office advises, such as burning where allowed or burying.
- 4
Wipe your pruning tools between cuts.
- 5
Check wild cherry and plum trees nearby, because they can be a source of spores.
- 6
Ask your extension office whether a fungicide program makes sense for a high-value tree.
Safety first: do not climb, do not cut near power lines and do not use a chainsaw on storm-loaded wood. Call the utility for downed lines and 911 if anyone is in danger.
Prevention
Prune out knots every winter, keep the tree growing well and remove neglected wild cherry and plum trees close by. Some plum varieties are listed as more resistant, so ask a nursery or extension office. Fungicide sprays at bud break are sometimes used with pruning, but check the label and local advice first.
When to call a pro
Call a certified arborist if knots are on the trunk or on large limbs, if a tree is heavily infected, or if the limbs are high or near wires. A heavily infected tree may be better removed than sprayed for years.
Not sure who to call? Read how to hire a tree service or check is my tree dangerous?
Trees it affects
Usually seen on: Black cherry, Plums and other cherry trees, Other Prunus (stone fruit) trees and shrubs.
See all species in our tree library, or pick a tougher tree with the tree picker.
For tree companies
How to explain black knot to a customer
Explain that pruning is the first and most important step, and that winter is the best time to see the knots. Be honest that a heavily infected tree may not be worth saving. Give a clear, written choice between pruning, a spray plan and removal.
Send this page to the customer after your visit so they can read the signs in plain words. See also selling plant health care and the job photo checklist.
Questions
What is black knot?
It is a fungal disease that makes hard, black, swollen growths on the twigs and branches of cherry and plum trees. The knots grow bigger each year.
Can black knot kill a tree?
It can. The knots can girdle and kill branches, and heavy infections can stunt or kill the whole tree if nothing is done.
When should I prune black knot?
The knots are easiest to see in winter, and many extension sources advise pruning when the tree is dormant. Cut well below the knot into healthy wood.
What do I do with the cut knots?
Take them off the site and dispose of them as your extension office advises. Leaving them on the ground can let spores spread.
Do I need fungicide?
Not always. Pruning is the main control. Some sources list sprays as an extra step for trees that get it often. Ask your extension office.
Will it spread to my neighbor's trees?
Spores can travel on wind and rain to other cherry and plum trees. Pruning knots and removing infected wild trees helps.
Can Lirevon diagnose black knot on my tree?
No. Lirevon is a remote marketing team for tree service companies and does not do tree work. This page is general information, not a diagnosis. A certified arborist or your county extension office can confirm what is wrong with your tree.
How do I find a good local pro for this?
Search Google Maps for certified arborists or tree services near you, read their reviews and ask for proof of insurance. Get two or three written quotes. Our guide on how to hire a tree service lists the questions to ask.
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