Black Knot on Cherry and Plum Trees in Rochester: The Black Swollen Galls Explained

By Linda Marsh, Pests & Diseases. Last updated: June 25, 2026

If you have noticed hard, charcoal-colored growths wrapped around the twigs of an ornamental cherry, a flowering plum, or a backyard fruit tree, you are almost certainly looking at black knot. It is one of the most common and most disfiguring diseases on Prunus trees across Monroe County, and it spreads readily in our cool, wet Finger Lakes springs. The good news: it is manageable with the right cuts at the right time of year.

What does black knot actually look like?

Black knot is caused by the fungus Apiosporina morbosa (older references call it Dibotryon morbosum). The signature symptom is a knobby, elongated gall that swells around twigs and branches. In its first year the swelling is often soft and olive-green to brown, easy to miss. By the second year it hardens into the classic black, cracked, charcoal-like mass that gives the disease its name. Galls range from less than an inch to a foot or more along a branch.

The galls girdle and choke whatever they encircle. Twigs beyond a knot wilt, leaf out poorly, or die back entirely. Over several seasons an untreated tree develops dozens of galls, loses limbs, and slowly declines. On ornamental flowering cherries and plums, the damage is both structural and aesthetic.

Which Rochester trees get black knot?

Black knot attacks members of the Prunus genus. In Monroe County yards that means:

• Ornamental and flowering cherries (including the popular Kwanzan and weeping types)
• Flowering and purple-leaf plums
• Backyard plum and cherry fruit trees
• Wild and naturalized black cherry (Prunus serotina), which grows along fence lines, woodlots, and roadsides all over Upstate New York

That last one matters enormously. Wild black cherry is everywhere in our region, and an infected wild tree on a neighbor's property or in the woods behind your fence acts as a constant reservoir of spores. This is a big reason black knot is so hard to fully eradicate here, and why it behaves a lot like other regional spore-driven diseases such as cedar apple rust that rely on a nearby alternate host.

How does black knot spread in our wet springs?

The fungus overwinters inside the galls. In spring, usually from bud break through early summer, warm rains trigger the galls to release spores. Those spores travel on wind and splashing water and infect new green shoots, especially during the wet stretches that lake-effect weather hands us in April and May. A run of 50-plus-degree rainy days in spring is when most new infections land.

The infection you see this year is the result of spores released a year or more ago. That lag is why people think they "got rid of it" after one round of pruning and then watch new knots appear the following season. Plan on managing black knot for several years, not fixing it in one.

How do you treat black knot the right way?

The core of black knot control is sanitation pruning, and the details matter:

1. Prune in the dormant season. Cut in late winter, ideally February into early March, before bud break and before spores are flying. Dry, cold conditions reduce the chance of spreading the fungus while you work. Dormant pruning is the same window that benefits most of your trees, which is worth coordinating with your broader plan for ongoing plant health care.

2. Cut 6 to 8 inches below each gall. The fungus extends well past the visible swelling. A cut right at the edge of a knot leaves infected wood behind, and the gall simply regrows. Removing 6 to 8 inches of clean wood below the knot is the single most important step.

3. Disinfect your tools between cuts. Wipe blades with rubbing alcohol or a 10% bleach solution between cuts to avoid moving spores from one branch to the next.

4. Destroy the prunings. Do not toss galled wood in a brush pile or leave it on the ground. Galls keep releasing spores even after they are cut off. Burn, bury, or bag and remove all infected wood.

5. Treat large trunk galls carefully. When a knot has formed on a main scaffold limb or the trunk, pruning it off may not be an option. These cases need professional judgment about whether the tree is worth saving.

Fungicides can offer some protection on high-value ornamentals, but timing is exacting (applications target the spring infection window) and they never substitute for removing existing galls. Cutting out the galls does the real work. Sprays only help around the edges.

Why does black knot keep coming back?

Three reasons account for most reinfections in Monroe County:

• Incomplete cuts. Pruning too close to the gall leaves fungus in the branch.
• Wood left on site. Cut galls still release spores; they must leave the property.
• A nearby reservoir. That untreated wild black cherry along the back fence, or an infected ornamental in the next yard over, keeps showering your trees with fresh spores every wet spring.

You can do everything right on your own tree and still see new knots if a spore source sits next door. The realistic goal in our region is ongoing management, not one-and-done eradication. This stubborn, reinfecting pattern is something black knot shares with bacterial diseases like fire blight on pear and crabapple, where a single overlooked source can undo a season of careful work.

When should you call an arborist for black knot?

Call a professional when galls have spread into the upper canopy beyond ladder reach, when knots have formed on the trunk or major scaffold limbs, when a treasured ornamental has heavy infection, or when you simply want the diagnosis confirmed before cutting. Homeowners often mistake young black knot for storm damage or insect galls, and an ISA Certified Arborist can tell black knot from other cankers and decide whether the tree is salvageable or a candidate for removal.

FAQ

Will black knot kill my cherry or plum tree? It can over time. Black knot rarely kills a tree in a single season, but unchecked galls girdle more branches each year, causing progressive dieback and eventual decline of heavily infected ornamentals.

Can I just cut the black knots off in summer? It is far better to wait for the dormant season (late winter into early March). Pruning during the wet spring and summer spore-release window risks spreading the fungus to fresh cuts and new shoots.

How far below the gall do I cut? Cut 6 to 8 inches below the visible knot. The fungus grows well beyond the swelling, so a cut right at the gall leaves infected wood and the knot regrows.

Is black knot contagious to my apple or maple trees? No. Black knot only affects Prunus species (cherries, plums, and related trees). Your apples, crabapples, and maples are not at risk from it, though they face their own regional diseases.

Why do the knots keep coming back after I prune? Usually because cuts were too shallow, galled wood was left on site, or an untreated wild black cherry or neighboring ornamental is reseeding your tree with spores each spring.

Sources

• Monster Tree Service of Rochester
• Cornell Plant Disease Diagnostic Clinic
• Cornell Cooperative Extension
• Penn State Extension, Black Knot of Prunus
• USDA Forest Service