Black Knot Disease: Treatment and Ongoing Care
Black knot disease is a fungal infection that affects cherry and plum trees, caused by Apiosporina morbosa. It is characterized by the appearance of black, swollen, and elongated growths, or "knots," that form on branches and twigs. These knots can restrict the flow of nutrients and water, leading to the decline of the affected tree. As the disease progresses, it can cause branches to die and potentially kill the tree if left untreated.
To treat black knot disease, infected branches should be pruned away during the dormant season, ideally in late winter or early spring, to minimize the spread of spores. It is essential to sterilize pruning tools between cuts to prevent cross-contamination. Fungicide applications may help manage the disease, especially in areas with high humidity or frequent rainfall. Regular monitoring and early intervention are crucial in preventing further spread and damage. We do not recommend homeowners diagnose or treat Black Knot but call Seacoast Gardener for a free consultation and diagnosis.
What Black Knot Looks Like
Black knot often begins as small olive-green swellings on young branches in early stages. Over time, these swellings harden into the rough, black, elongated galls that are most commonly recognized.
These growths can start on smaller twigs and gradually expand to larger branches if left untreated. A single untreated knot can release thousands of spores during the growing season, spreading the infection to nearby trees.
3 Key Steps for Treating Black Knot:
Prune Infected Branches: Identify and remove any affected branches or twigs, which appear as black, swollen lesions. Use sharp, sterilized pruning tools to cut back infected areas at least 4 to 6 inches below the knot to prevent spreading.
Dispose of Infected Material: After pruning, properly dispose of the infected branches by burning or bagging them. Avoid composting, as the disease can persist in the material and spread back to healthy trees.
Apply Fungicide: Use a fungicide labeled for black knot disease to protect healthy trees. Apply during the early spring or fall before the infection spreads, following the manufacturer’s instructions carefully for effective results.
When Removal May Be Necessary
In cases where black knot has spread to large branches or multiple areas of the tree, pruning alone may not be enough.
Heavily infected trees can continue spreading spores to nearby plants and may become a long-term issue in the landscape. In such cases, full removal and replacement may be the most effective solution.
Top Fungicides for Black Knot:
Here are some of the best fungicides used by professionals in the region:
1. Chlorothalonil
Trade Names: Daconil WeatherStik, Bravo
Why it works: Chlorothalonil is a broad-spectrum protectant fungicide effective against many fungal diseases, including black knot. It's most effective early in the season during bud break through petal fall, when infection typically occurs.
Pro tip: This is a contact fungicide, so reapplication is crucial after rain or rapid new growth.
2. Thiophanate-methyl
Trade Names: Cleary’s 3336, Bonide Infuse
Why it works: Systemic fungicide that penetrates plant tissue and offers both protective and curative action. Often used in rotation with Chlorothalonil to manage resistance.
Pro tip: Ideal for commercial orchards or high-value landscape trees when combined with cultural controls.
3. Propiconazole
Trade Names: Banner Maxx, Bonide Fung-onil Lawn & Garden
Why it works: Highly effective as a systemic fungicide and often used in professional orchard management. It inhibits fungal growth from within the plant, offering longer protection windows.
Pro tip: Use this fungicide sparingly and rotate with other modes of action to avoid resistance development.
4. Copper-based fungicides
Trade Names: Copper Soap, Bordeaux Mix, Bonide Liquid Copper
Why it works:
Provides broad-spectrum disease suppression and is more organic-friendly. Best applied before bud break as a dormant spray. Less effective during rapid infection periods but helps reduce initial inoculum.
Pro tip: Repeated use can cause copper accumulation in the soil, so use judiciously.
Which Trees Are Most at Risk
Black knot primarily affects trees in the Prunus family. The most commonly affected include:
• Plum trees
• Cherry trees (both ornamental and fruiting)
• Wild black cherry and chokecherry
Some trees, such as apple, pear, peach, and hydrangea, are not significantly affected.
If wild cherry or plum trees are growing nearby, they can act as a constant source of reinfection, making management more difficult.
Important Note on Fungicides
Fungicides can help prevent new infections, but they do not cure existing black knot growths.
This is why proper pruning and removal of infected material is the most critical step in treatment. Fungicides should only be used as a preventive measure, especially during early spring when new growth is most vulnerable.
Ongoing Care and Prevention
Black knot requires consistent monitoring over time. New infections can take a year or more to become visible, which is why annual inspection is essential.
We recommend checking trees during late winter each year and removing any new growths before the growing season begins.
Managing nearby wild host trees and maintaining proper pruning practices can significantly reduce the risk of reinfection.
Need Help Treating Black Knot?
Black knot can spread quickly and become difficult to manage without proper care.
If you suspect your tree may be infected, contact Seacoast Gardener for a professional assessment and treatment plan. Early intervention can make the difference between saving and losing a tree. Contact Us Now: 📞 (603) 770-5072 | 🌐 www.seacoastgardener.com
Download Our Complete Black Knot Guide
For a more detailed breakdown of treatment timing, plant types, and expert recommendations, download our full guide: