Organic Pest & Disease Control

Look for Enlarged Growths on Your Cherry Trees in Winter

Who doesn’t appreciate the burst of color ornamental cherry trees bring to the springtime?

Many look forward to the stunning displays of light pink blossoms that give the effect of light snow when falling from the tree. On average, ornamental cherry trees live between 15-20 years. But their lives can be much shorter lived due to a highly contagious tree disease that threatens to eliminate the existence of these springtime showstoppers. And it’s not just ornamental cherry trees this fungus targets. It also attacks plum and chokecherry trees.

This insidious disease is called Black Knot Fungus. The best time to look for early signs of Black Knot or Apiosporina Morbosa is in the winter. Sounds morbid? Well, it is.

Close up of Black Knot Fungus on a cherry tree.

Types of Trees Susceptible to Black Knot Fungus

If you have the following trees on your property, be on the lookout for an insidious fungus during the winter months. It’s almost impossible to spot this tree disease in the spring and summer months.

Latin NameCommon Name
Prunus americanaAmerican Plum
Prunus aviumMazzard Cherry
Prunus armeniaca var. mandshuricaManchurian Apricot
Prunus besseyiWestern Sand Cherry
Prunus maackiAmur Chokecherry
Prunus padusEuropean Bird Cherry
Prunus pennsylvanicaPin Cherry, Wild Red Cherry
Prunus serotinaBlack Cherry
Prunus tomentosaManchu Cherry
Prunus virginianaCommon Chokecherry, Schubert Cherry
This stunning tree fell victim to this dreaded cherry tree disease. The photos in this post are from when this tree was infected with Black Knot Fungus.

Why this Tree Disease Should be Taken Seriously

It’s a fungus that attacks Prunus species like cherry, plum, apricot and chokecherry trees. From my research, the first recorded case was in 1821 in Pennsylvania but has since spread through North America. It targets both wild and cultivated Prunus species which is why it’s a serious problem. As the fungus progresses, it’s highly contagious and can wipe out populations of trees.

How Black Knot Attacks a Tree

You can see how many branches are affected during the winter when there are no leaves.

Black Knot spreads in the spring/summer through rain and wind. It gets into the tree through any wounded shoots or tissue. Black Knot overwinters in a dormant stage and spreads in the spring.

Treatment

It depends on how much the fungus has progressed. Hard pruning is highly recommended. Remove at least 4″-6″ of the branch around the knot. If the knots are on multiple branches, then remove the whole limb. However, if there are multiple branches affected, it’s better to cut the tree down and property dispose of it to avoid further spread.

First Signs

In the spring, it initially affects the woody parts of the tree like branches and twigs. Sometimes, it can grow from the trunk of the tree. Look for a spongy olive green growth on the branches. It can be hard to find when there are leaves in the trees which is why it’s recommended to inspect these trees in the winter because it will be easier to see. If you spot Black Knot, aggressively prune 4″-6″ of the branch. You can prune the tree in the winter when you see it. If there are multiple knots in the same area, remove the limb of the tree. Sanitize your pruning tools with 70% alcohol.

Please DO NOT compost infected branches and limbs. It is recommended that you burn it to kill the spores. If you discard the branches, it can continue to infect other areas. Continue to monitor the tree’s wooded branches and remove infected branches as soon as you see it. Dispose of the infected branches and limbs by burning them where it’s permitted. Please check with your local municipality on burning restrictions.

Keep watch in the next few winters to make sure there aren’t more growths.

The best time to spot black fungus is in the winter when the leaves have fallen off.

As it Progresses, You’ll See…

The growths or “galls” become more obvious the second winter of infection like the picture above. There are recommendations on other websites of spraying a fungicide. Even though some people recommended, it’s not always effective against Black Knot depending on whether or not the conditions are ideal for it to keep spreading.

It’s very important to me to avoid spraying any chemicals on my property because I am mindful of how chemicals upset the balance of nature specifically pollinators and water run off in our creek. The creek I live near is currently listed as a clean creek in our state. I would like to do my part to keep it that way.

When the Infection is Advanced…

You will see these type of growths on multiple branches of the tree especially in the winter when the leaves aren’t in the way. These knots may crack and ooze a sticky liquid. If there is a large knot growing from the trunk of the tree, then removing the whole tree and disposing it properly is recommended. If the Black Knot is advanced, it’s wise to remove the whole tree including the stump and burning it to remove spores. Again, please check local burning restrictions with your municipality.

Prognosis

If left untreated, Black Knot spreads to all of the branches eventually stunting and possibly killing the tree. It can also spread to neighboring yards and properties infecting neighboring fruit trees that are susceptible to this fungus.

Alternatives

If you’re adding new trees, you can find Prunus cultivars that are resistant to Black Knot. Click on the link to go to a plant database of fruit trees and look for the varieties that are resistant to Black Knot. It would be wise to wait a few years to make sure other trees in your area aren’t affected before planting the same variety of Prunus cultivars on the same property of the infected tree.