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.
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.
Top Fungicides for Black Knot in New England:
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.