Black knot fungus is a common tree disease and it’s caused by a fungus Apiosporina Morbosa. It mostly affects fruit trees like cherry and plum trees, both cultivated and wild. It manifests by creating and developing knobby dark or black growths on the branches and trunk. If left untreated, a black knot can result in the death of a tree. 

The Nature of Black Knot in Trees

black knot on tree in woods

This tree disease spreads during spring, as that’s the time of the year when it rains the most. It’s a perfect season for the fungus to release spores. The wind carries the spores and helps them find their way to the unhealthy branches and newer growths.

While outbreaks are most common for wild trees, they can spread to your yard if you don’t maintain your trees and your property. The trees that are most susceptible to black knot are weaker and younger trees. However, healthy, big, and older trees are able to resist it.

Dibotryon morbosum, the technical name for black knot fungus, is a fungal disease that mostly affects different types of Prunus trees, which includes ornamental cherry and plum trees. It manifests as rough, hard, black swellings or galls on the tree’s branches. These growths range in size from tiny knots the size of peas to massive, asymmetrical galls that skew the branches’ form.

Usually in the spring, the fungus infects the tree during its growth season and becomes dormant until the next spring. It is crucial to act quickly to stop its spread since it spreads by airborne spores and can live for years on infected branches.

The Symptoms of Black Knot

Typically, the black knot fungus causes noticeable growths on tree branches, particularly on Prunus species like cherry and plum trees. The following are the usual symptoms:

Galls or Knots: The development of stiff, tough, black swellings on the branches is the most obvious indication. The size of these growths can vary, ranging from tiny knots the size of peas to bigger, irregular galls. These knots may start soft and green while the fungus is active, but they will gradually solidify and turn black.

Branch Distortion: The branches frequently bulge or get twisted as a result of the growths. They are present on the tree’s main stem as well as twigs and branches.

Colour and Texture: The knots may seem olive-green or light brown at first, but they will soon solidify and darken, becoming tough and black. They are frequently misinterpreted as a component of the tree’s organic structure, which is why routine examination is essential.

Infection Spread: The fungus can grow throughout the branches, impacting several locations and weakening the tree over time and leaving it an unsightly eyesore.

Typically, these symptoms start in the spring and last the whole growth season. As time passes, the knots become more noticeable, which facilitates their identification and separation from the tree’s healthy sections.

How to Prevent and Treat Black Knot Fungus

house with trees and yardPrune Affected Branches: Pruning affected branches is the most efficient method of managing black knot fungus. When the tree lies dormant in winter is the ideal time to accomplish this. Make careful to sanitize your pruning tools with a mix of one part bleach to nine parts water in between cuts to stop the fungus from spreading. Cut at least 15 inches below the visible knot.

Destroy Infected Materials: Be sure to dispose of the trimmed branches properly. The best way to stop the spread of spores is to burn the contaminated material, but if that isn’t possible, double-bagging and throwing it in the garbage can also help.

Chemical Treatments: One preventative strategy is to apply fungicides. Follow the manufacturer’s directions and apply them early in the spring, before bud break, and all during the growth season. While fungicides cannot treat current diseases, they can shield the tree’s healthy sections from developing new ones.

Monitor and Maintain the Tree: Check your trees frequently for any indications of illness. It is possible to stop black knot fungus from spreading and reoccurring by pruning properly, leaving enough space between trees to allow for optimal ventilation, and maintaining general tree health with appropriate fertilizer and watering.

It’s crucial to remember that effective management of black knot fungus may need several years of constant work. To stop the illness from spreading to other trees in the area, it could also be prudent to think about cutting down and replacing the affected tree if it is serious and pervasive throughout the tree.

Speaking with a local agricultural extension agent or arborist can help you receive advice that is relevant to your area and the extent of the infection.


How Can Arborists Help With Tree Diseases & Pest Infestations?

an expert arborist executing tree pruning services for a clientProactive monitoring of tree diseases and pests is important to help maintain healthy trees on your property. How can arborists help?

  • Identify & Assess Potential Diseases & Infestations

  • Provide Appropriate Treatment For Your Trees

  • Monitor The Ongoing Health Of Your Trees

  • Provide Tree Care Tips To Avoid Complications

Want to make sure that you’re doing all you can for the trees on your property? Consider hiring professional and certified arborists. Our team of licensed arborists are equipped with the proper training, equipment, and experience to identify and treat your diseased trees. So contact us today for a consultation!

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