How to Treat Black Knot Fungus on Trees

Treating black knot fungus starts with aggressive pruning of every infected branch, followed by proper disposal of the removed wood and, in many cases, protective fungicide sprays timed to the period when the fungus actively releases spores. The disease, caused by the fungus Apiosporina morbosa, produces hard, swollen black galls on the branches and trunks of trees in the Prunus genus, and it will not resolve on its own. Left unchecked, it spreads steadily until the tree is disfigured, weakened, and sometimes killed. The good news is that a combination of physical removal and well-timed chemical protection can save most trees if you catch the problem before it overwhelms them.

Recognizing What You Are Dealing With

Black knot galls typically start as small, olive-green swellings on new-growth twigs in late spring or summer. By the following year they darken, harden, and elongate into the rough, coal-black masses that give the disease its name. Mature knots can range from an inch to over a foot long. They often encircle a branch, girdling it and killing everything beyond the gall. On some trees you might find just a handful of knots; on badly neglected specimens, every major limb can be studded with them.

The fungus attacks plums, cherries, and other stone-fruit trees. Research confirms that both Japanese and European plums are affected, and wild species like chokecherry serve as major reservoirs of the disease in surrounding landscapes.¹ Ornamental flowering plums and cherries commonly sold at nurseries are also susceptible. If you have a tree in the Prunus family with hard black growths on its branches, black knot is almost certainly the cause. No other common disease looks quite like it.

Pruning Is the Single Most Important Treatment

No fungicide alone will cure an existing knot. The gall is a mass of both fungal and plant tissue that has been permanently altered, and spraying it will not reverse the damage. The only way to remove active infections is to cut them out. This makes pruning the foundation of any treatment plan.

Cut infected branches at least four inches below the visible edge of each gall. Research using electron microscopy has found that the fungus can migrate through the wood’s internal vessels beyond the visible lesion in susceptible tree varieties, which is why you need that generous margin of healthy-looking wood.¹ If a knot sits on a major scaffold limb or the trunk where you cannot cut four inches below it, you can try carefully removing just the gall and a margin of surrounding bark and wood, though success rates drop when the infection is on large-diameter wood.

A few practical tips make the pruning more effective:

Prune in winter: Dormant-season pruning is ideal because you can see the galls clearly against bare branches, and the fungus is not actively releasing spores. Late winter, before bud break, is the sweet spot.
Sterilize tools between cuts: Wipe your pruning saw or loppers with rubbing alcohol or a dilute bleach solution after each cut to avoid dragging fungal material from one wound to the next.
Dispose of removed wood immediately: Do not leave pruned galls on the ground under the tree. Bag them and send them to the landfill, or burn them where local regulations allow. Black knots left on the ground can continue producing spores for months.

If your tree has so many knots that removing them all would mean cutting away more than half its canopy, the tree is probably too far gone. At that point, removal of the entire tree is the more realistic option, both for the tree’s own sake and to protect nearby healthy trees from a constant shower of spores.

Timing Fungicide Applications to Spore Release

Fungicides play a supporting role. They cannot cure existing infections, but they can protect new growth from becoming infected during the window when the fungus actively spreads. That window matters a lot. Research on sour cherry trees found that peak ascospore release occurred from late May through the end of June, a period that coincides roughly with the shuck-split stage through early shoot elongation. Spore release depended on rainfall and temperature but not on how long surfaces stayed wet.² This means the critical protection period is spring, particularly when rain events coincide with warm temperatures and new shoots are actively growing.

The most commonly recommended fungicides for black knot contain chlorothalonil, captan, or a combination of lime sulfur applied before bud break. For home orchardists, products labeled for stone-fruit disease control that contain chlorothalonil are typically the most accessible. Commercial plum growers often use more targeted options. In Canadian plum production, roughly two-fifths of variable costs go toward pruning and fungicide applications, which gives a sense of how seriously commercial operations take the disease.³

A typical homeowner spray schedule looks something like this: apply a dormant-season lime-sulfur spray before bud break, then switch to chlorothalonil or captan at bud break and reapply every seven to ten days through the end of June, especially if rain is frequent. Once new shoots have hardened off and the peak spore-release window closes, you can stop. The fungicide label will specify exact rates and intervals; follow it closely. Spraying outside the critical window wastes money and chemical without reducing infection risk.

Why Some Trees Get Hit Harder Than Others

Not all Prunus species and varieties are equally vulnerable. Among plums, some cultivars show genuine resistance while others are devastated. A study that phenotyped 200 Japanese plum trees for black knot severity over multiple years found that the proportion of completely resistant trees stayed constant between 2018 and 2023, suggesting the resistance was stable and real rather than a fluke of low disease pressure in a single season.⁴ Breeding programs are actively working to identify the genetic basis of that resistance so it can be incorporated into commercial varieties more reliably.

Among European plums, researchers have compared how tolerant and susceptible cultivars respond anatomically to infection. In susceptible varieties, the fungus aggressively colonizes internal tissues and can move through the vascular system to areas beyond the visible gall. In tolerant varieties, the tree mounts a stronger compartmentalization response that limits fungal spread.¹ This means that the “cut four inches below the gall” rule is especially critical for susceptible varieties, where the fungus may be lurking further from the visible infection than you would expect.

If you are planting new plum or cherry trees and black knot is present in your area, choosing a resistant or tolerant cultivar is probably the single best long-term investment you can make. Among plums, ‘President’ and ‘Italian Prune’ have historically been cited as more tolerant European types, while many ornamental purple-leaf plums are notoriously susceptible. Among cherries, tart cherry varieties tend to be more susceptible than sweet cherries, though no cherry is completely immune. Ask your local extension office which cultivars perform best in your specific region, because disease pressure and climate both influence outcomes.

The Problem with Nearby Wild Trees

This is where many homeowners hit a wall. You can prune every knot off your own trees, spray on schedule, and still see new infections appear the following year. The reason is usually wild Prunus trees in the surrounding landscape. Wild chokecherries and wild plums are extraordinarily common across much of North America, and they harbor the fungus with no one managing them. Research on the genetic diversity of Apiosporina morbosa has found that wild chokecherry populations produce more unique fungal genotypes and higher genetic diversity than populations on cultivated varieties, and there is significant gene flow between wild and cultivated populations.⁵ In plain terms, the wild trees serve as a diverse breeding ground for the fungus, and spores travel easily to your yard.

If you own the surrounding property, removing wild Prunus trees within a few hundred feet of your orchard or garden trees reduces spore pressure considerably. If the wild trees are on neighboring land or public spaces, you obviously cannot remove them, but you can factor their presence into your management strategy. It means fungicide sprays become more important, not optional, and you should inspect your trees every year for new infections rather than assuming last year’s pruning solved the problem permanently.

Some municipalities in areas with heavy black knot pressure have bylaws requiring property owners to remove infected wild trees or at least prune visible knots. If you are in a neighborhood where every third lot has a neglected chokecherry full of black galls, it may be worth checking whether your local government has any such regulations and encouraging enforcement.

What the Infection Timeline Actually Looks Like

One of the things that trips people up with black knot is the lag between infection and visible symptoms. The fungus infects new green shoots in spring, but you will not see anything unusual until the following growing season. That first year, all that appears is a slight greenish or brownish swelling that is easy to overlook. The hard black gall does not form until the second year after infection. By the time you notice a conspicuous black knot, the infection is already at least a year old, and the tree may have been reinfected in the current season too.

This lag is why a one-and-done pruning job rarely solves the problem. After your initial cleanup, you need to come back the following winter and check for new galls that were in their invisible first-year stage when you pruned. Most arborists recommend a minimum of two to three consecutive years of diligent pruning and spraying before you can consider the disease controlled. Even then, if there are wild hosts nearby, annual inspections remain necessary.

Spore release peaks in late May through June on cherry trees, and the infection window lines up with warm, rainy weather during active shoot growth.² A wet spring is a high-risk spring. Conversely, a dry May and June can reduce new infections substantially even without fungicide. You cannot control the weather, but understanding this pattern helps you gauge how aggressive your spray program needs to be in any given year. If it has been raining every other day through June, stay on your spray schedule; if conditions have been dry, you have more margin for forgiveness.

Common Mistakes That Let the Disease Come Back

People often approach black knot treatment with good intentions and then undermine themselves in a few predictable ways. Understanding these pitfalls saves you from wasting effort.

Cutting too close to the gall: If you trim right at the edge of the visible knot, you almost certainly leave fungal tissue behind. The four-inch margin below the gall exists because microscopic analysis shows the fungus extending beyond what you can see, especially in susceptible cultivars.
Leaving pruned galls on the ground: A removed knot sitting under the tree or in a brush pile keeps producing spores. It is not dead just because you cut it off the tree. Bag it, burn it, or get it off your property.
Spraying only once: A single fungicide application provides a narrow window of protection. During a rainy spring, new growth emerges continuously and spores release over weeks. Reapplication on the label’s schedule is what actually reduces new infections.
Stopping after one year: Because of the one-to-two-year lag between infection and visible gall formation, infections you cannot yet see at the time of your first pruning will show up later. You need to return the following winter to catch them.
Ignoring small galls: A knot the size of a pea is still producing fungal tissue internally and will grow. Small galls are actually easier and less damaging to remove than large ones. Do not wait for them to get worse.

When Removal Is the Better Call

There comes a point where treating a heavily infected tree is not worth the effort. If galls cover the main scaffold limbs and trunk, the tree’s structural integrity is compromised and its fruit production (if it is a fruiting variety) has collapsed. Keeping a severely infected tree in the landscape also turns it into a spore factory that endangers every other Prunus within a wide radius.

A reasonable threshold is this: if you would need to remove more than about half the canopy to cut out all visible infections, and additional galls appear on the trunk or major crotches, the tree is unlikely to recover into a productive or attractive specimen. Removing it and replanting with a resistant cultivar is the faster path to a healthy tree. When you do remove it, grind or remove the stump, because Prunus stumps sometimes send up suckers that can become newly infected.

Organic and Low-Chemical Options

If you prefer to avoid synthetic fungicides, your options are more limited but not nonexistent. Lime sulfur, applied as a dormant spray before bud break, is one of the oldest fungicidal treatments and is accepted in many organic programs. It is caustic and unpleasant to work with, and it can damage foliage if applied after green tissue emerges, so timing has to be precise. Copper-based fungicides are another possibility, though their effectiveness against black knot specifically is less well documented than chlorothalonil.

The honest reality for organic management is that pruning and sanitation carry even more of the load. Without a reliable protectant fungicide through the spring infection window, you are depending almost entirely on removing every source of inoculum and hoping your cultivar has decent tolerance. In a landscape surrounded by infected wild trees, that is a tough position. For organic growers in high-pressure areas, choosing a resistant cultivar is not just helpful but nearly essential. Cultural practices like maintaining good air circulation through the canopy (so foliage dries quickly after rain) and avoiding overhead irrigation during the critical spring period also help at the margins, though they are supplements to pruning rather than substitutes for it.

How Black Knot Differs from Other Tree Galls

Not every lump on a tree branch is black knot. Insect galls, bacterial crown gall, and various cankers can all produce swellings that an anxious tree owner might mistake for the disease. Black knot galls have a distinctive texture and progression that set them apart. They are hard and woody, not spongy or corky. They start as greenish swellings and mature to a rough, charcoal-black surface. In spring, the surface of mature galls sometimes has a velvety olive-green layer of actively sporulating fungal tissue before it dries and blackens again.

Insect galls, by contrast, are often smooth, round, and sometimes brightly colored. Bacterial crown gall tends to appear near the soil line or at graft unions and has a more irregular, cauliflower-like texture. Cankers are usually sunken or flattened, not raised and swollen. If you are unsure, your local cooperative extension service can usually identify the problem from a photo. Getting the diagnosis right matters because the treatment for insect galls (often nothing) and bacterial gall (removing the affected tree) is quite different from the pruning-and-fungicide approach that works for black knot.