If you’ve noticed dark, rough growths on your tree’s branches, you might be dealing with black knot on tree. This harmful fungal growth is a serious disease that can weaken and even kill susceptible trees if left untreated.
The good news is that with proper identification and timely action, you can manage this problem. This guide will walk you through everything you need to know, from spotting the early signs to executing an effective removal and prevention plan.
Black Knot On Tree
Black knot is caused by the fungus Apiosporina morbosa. It primarily targets trees in the Prunus genus. This includes many fruiting and ornamental species that are common in home landscapes.
The fungus forms distinctive galls, or “knots,” on branches, twigs, and sometimes even the trunk. These start small but can grow to over a foot in length, girdling and choking off the tree’s vascular system. This prevents the flow of water and nutrients.
Which Trees Are Most at Risk?
Not all trees are susceptible. The fungus has a clear preference. You should be especially vigilant if you have any of the following:
* Fruit Trees: Plum and cherry trees (both fruiting and ornamental) are the most common and hardest-hit victims. This includes American, European, Japanese, and flowering varieties.
* Ornamental Trees: Purple-leaf plum, chokecherry, and Mayday tree are highly susceptible.
* Wild Trees: Native black cherry and pin cherry can also host the fungus, often serving as a source of infection for cultivated trees nearby.
If you have any of these species on your property, regular inspection is your first and best line of defense. Peach, apricot, and nectarine trees are less commonly affected but are not completely immune.
The Life Cycle of the Fungus: Knowing Your Enemy
Understanding how black knot spreads is key to stopping it. The fungus has a two-year life cycle that’s important to grasp.
In the first spring, infection occurs. The fungus releases microscopic spores from mature knots on an infected tree. These spores are carried by wind and rain to new, healthy branches.
Infection requires fresh, green growth. The spores enter through young shoots or through wounds in the bark. They germinate and begin to grow inside the wood, where you can’t see them.
By the fall or following spring, the first visible signs appear. Look for small, olive-green, corky swellings on the new twigs. They are often slightly swollen and may feel velvety. This is the critical early stage.
In the second year, these swellings harden and turn black. They grow rapidly, elongating along the branch. By the second summer, they begin producing spores themselves, ready to spread the disease to other parts of the tree or to neighboring trees. A mature knot can release spores for several years.
How to Identify Black Knot Disease
Correct identification is crucial, as it can be confused with burls or other growths. Here’s what to look for:
* Early Stage (Year 1): Look for small, olive-green to brown swellings on current or previous year’s growth. They are often slightly elongated. They may be easy to miss without close inspection.
* Late Stage (Year 2+): The knots become hard, brittle, and coal-black. They are rough and cracked, like charcoal. They can range from an inch to over a foot in length.
* Location: Found primarily on branches and twigs. In severe cases, they form on trunks and even on the tree’s main scaffold limbs.
* Effect on Tree: Branches beyond the knot will often wilt, die back, and become stunted. Leaves may be smaller and fewer. Severe infections lead to a general decline in the tree’s health, making it vulnerable to other pests and winter damage.
Step-by-Step Removal and Control
Once you’ve identified black knot, immediate action is required. The primary method of control is physical removal, or pruning. Fungicides play a supporting role. The best time to prune is in late winter or very early spring, before the tree breaks dormancy. At this time, the fungus is inactive, and spores are not being released. It’s also easier to see the knots without leaves in the way.
Step 1: Gather the Right Tools
You’ll need a few basic tools. Make sure they are sharp and clean:
* Bypass pruners for small twigs (under ¾ inch).
* Loppers for medium branches (¾ to 1 ½ inches).
* A pruning saw for larger branches.
* A disinfectant solution (like 70% isopropyl alcohol or a 1:9 bleach-to-water mix).
* A sturdy tarp or sheet to collect debris.
* Heavy-duty garbage bags.
Step 2: Pruning Out the Infection
Follow these steps carefully for effective removal:
1. Inspect the Entire Tree. Start from the bottom and work your way up, examining every branch. Mark infected ones with string if needed.
2. Make Your Cuts. For each infected branch, you must cut well below the visible knot. The fungus extends far beyond the obvious gall. Cut at least 6 to 8 inches below the knot, and preferably back to the next branch union or the main trunk. Make sure your cut is into clean, healthy, green wood.
3. Disinfect Between Every Cut. This is critical. Dip or wipe your pruning tool blades in your disinfectant solution after every single cut. This prevents you from spreading the fungus on your tools as you work.
4. Collect All Debris. As you prune, let the infected branches fall onto your tarp. Do not let them touch the ground. This contains the spores.
Step 3: Disposal and Cleanup
How you dispose of the material is just as important as removing it. Never compost black knot debris.
The fungal spores can survive and reinfect your garden. The best method is to burn the material if local ordinances allow. If burning isn’t an option, seal the debris in heavy-duty plastic bags and send it to the landfill. Also, be sure to rake up and dispose of any fallen knots from under the tree.
Step 4: When to Consider Fungicides
Pruning is the main solution, but fungicides can help protect new growth. Use them as a protective shield, not a cure. They are most effective for high-value trees or in areas with severe, recurring infections.
Apply a fungicide labeled for black knot on fruit trees. Look for active ingredients like chlorothalonil or thiophanate-methyl. Timing is everything:
* Make the first application when buds first break open in spring.
* Reapply according to label directions, typically every 7-14 days during wet spring weather, until the new shoots have fully expanded and hardened off.
* Thorough coverage of all new green tissue is essential.
Remember, fungicides won’t eliminate existing infections. They only protect new growth from spore germination. Always combine fungicide use with a rigorous pruning program.
Long-Term Prevention and Tree Care
Preventing black knot is far easier than fighting a severe infection. A healthy tree is also more resilient. Here’s your long-term strategy:
* Choose Resistant Varieties: When planting new trees, seek out disease-resistant cultivars. Some plum varieties, like ‘President’, show better resistance.
* Annual Inspection and Pruning: Make it a late-winter ritual. Inspect all susceptible trees and prune out any new knots immediately. This is the single most effective practice.
* Maintain Tree Health: A stressed tree is more suseptible. Water your trees deeply during droughts. Apply a layer of organic mulch around the base (but not touching the trunk) to conserve moisture and improve soil health. Avoid excessive nitrogen fertilizer, which promotes the succulent growth the fungus loves.
Manage the Surroundings: Check wild Prunus trees (like chokecherries) in nearby hedgerows or woodlands. If they are heavily infected, consider removing them if possible, as they are a constant source of spores.
Frequently Asked Questions (FAQ)
Q: Can a tree with black knot be saved?
A: Yes, in most cases, if the infection is caught early and not on the main trunk. Consistent, careful pruning over several seasons can often control the disease and restore the tree’s health.
Q: Is black knot fungus harmful to humans or pets?
A: The fungus itself is not toxic to touch, but the hard, brittle knots can break off and become a physical hazard. Always wear gloves when handling infected material, as the moldy knots can cause irritation for some people.
Q: Can I use the wood from a tree killed by black knot?
A: It’s not recommended for firewood if there are other susceptible trees nearby, as the knots can still release spores. If you do use it, burn it completely and promptly. The wood is otherwise safe for crafting once the knots are removed and the wood is dried.
Q: What does black knot look like when it first starts?
A: It begins as small, olive-green, slightly swollen or “corky” areas on young twigs. They are often soft and may have a velvety texture. This stage is easy to overlook but is the best time to prune it out.
Q: How does black knot disease spread?
A: It spreads primarily by wind and rain. Spores from mature black knots are released during wet weather in spring and are carried to new, susceptible green growth on the same tree or neighboring trees.
Managing black knot requires patience and diligence. It’s not a one-time fix but an ongoing part of caring for susceptible trees. By staying vigilant with your annual inspections and pruning, you can protect your trees from this harmful fungal growth and enjoy their beauty and bounty for years to come.