Black knot on plum and cherry

Black knot is one of the most visually distinctive plant diseases in the eastern US -- the hard, black, corky, elongated galls on plum and cherry branches are unmistakable once you've seen them, and in neglected plantings they can cover branch after branch until the tree is structurally.

—- title: "Black knot on plum and cherry" slug: black-knot-on-plum hub: problems category: "Disease-by-host" description: "Black knot creates hard, black, corky growths on plum and cherry branches. Learn to identify the early olive-green stage, prune correctly, and choose resistant cultivars to prevent reinfestation." date: 2026-06-10 updated: 2026-06-10 author: "Thomas A." reading_time: 8 —-

Black knot is one of the most visually distinctive plant diseases in the eastern US — the hard, black, corky, elongated galls on plum and cherry branches are unmistakable once you've seen them, and in neglected plantings they can cover branch after branch until the tree is structurally compromised. The disease is caused by a native North American fungus and spreads readily from wild plums and cherries (including ornamental species) that grow throughout the region.

I don't grow plums or cherries at my Long Island property, so this guide draws on Cornell Cooperative Extension, Penn State Extension, and Clemson HGIC research.

The pathogen

Black knot is caused by Apiosporina morbosa (formerly Dibotryon morbosum). Per Cornell Cooperative Extension, the fungus is native to North America and infects primarily Prunus species — plums, cherries, and closely related trees.

The disease is endemic to the eastern US, where wild black cherry (Prunus serotina) and chokecherry (P. virginiana) serve as reservoirs. Per Penn State Extension, spores are released from existing knots in spring when temperatures exceed 55°F (13°C) and there is rain. They infect succulent green shoots of the current season.

Identification

Early stage (year of infection)

Per Penn State Extension, black knot does not become visible the same year the infection occurs:

1. Olive-green swellings appear on infected shoots in late summer to fall of the year of infection — these are small, elongated, soft swellings, easily missed
2. By the following spring, these swellings enlarge and begin producing the corky, black growth

Mature knots

Per Clemson HGIC, mature black knot produces:

• Hard, corky, black galls ranging from 0.5 inch to several inches long, encircling the stem
• Surface that starts olive-brown in the first year and becomes jet black as the knot matures over 2–3 years
• A velvet-like surface texture produced by the masses of asci and spores on the knot surface
• Girdling of the branch above the knot — tissue above a fully formed knot dies from loss of water and nutrient transport

Susceptibility by host

Per Cornell Cooperative Extension, susceptibility varies:

• European plum (P. domestica) — highly susceptible; cultivars Stanley, Italian Prune, and others are severely affected in high-pressure areas
• Japanese plum (P. salicina) — variable; some cultivars more resistant
• American plum (P. americana) — variable
• Sweet cherry (P. avium) — susceptible but generally less severe than on plums
• Sour cherry (P. cerasus) — susceptible
• Ornamental cherries and plums — many cultivars are highly susceptible and serve as inoculum sources in suburban landscapes

Disease cycle

Per Penn State Extension:

1. Mature knots on branches overwinter and produce spores in spring (April–May in the mid-Atlantic and Northeast)
2. Spores are splash-dispersed and infect succulent new shoots from bud break through about 6 weeks after bud break
3. Infection is invisible through summer
4. Olive-green swellings appear on infected shoots by late summer; these are the first-year knots
5. Over the next 1–2 years, knots grow, blacken, and become the corky, spore-producing structures that continue the cycle

The 1-year lag between infection and visible symptom is important: when you see a mature black knot, the infections from 1–2 years of spread are already in the tree as invisible or olive-stage knots. Aggressive pruning must address all stages.

Management

Pruning — the primary management tool

Per Clemson HGIC, prune out all knots during late winter while the tree is dormant and the knots are most visible. Cut a minimum of 4–6 inches below the visible base of each knot — the fungal mycelium extends into the branch tissue beyond the visible swelling, and cuts made at the knot edge frequently leave behind infected tissue that regrows.

Disinfect pruning tools between each cut with 70% isopropyl alcohol or a 10% bleach solution. Bag and dispose of prunings in the trash — do not compost or chip them.

Per Penn State Extension, knots on the main trunk or major scaffold branches that cannot be removed without destroying the tree structure require a different approach: use a chisel to remove all infected bark and wood tissue from the knot, cutting back to clean white wood, then paint the wound with a protective paint or copper paste to reduce reinfection. This "carving" approach is less effective than removal but preserves tree structure when necessary.

Fungicide applications

Per Cornell Cooperative Extension, protective fungicide applications during the spring infection period (bud break through 6 weeks after bud break) reduce new infections. Registered active ingredients include:

• Thiophanate-methyl — effective; apply every 7–10 days from bud break through early summer
• Propiconazole — effective systemic fungicide
• Myclobutanil — effective for the spring infection window
• Copper-based fungicides — organic option; apply every 7–10 days

Fungicides do not eliminate existing knots — they prevent new infections. Combine fungicide with thorough pruning for best results.

Remove nearby wild hosts

Per Penn State Extension, wild black cherry and chokecherry trees within 600 feet of cultivated plums and cherries are significant inoculum sources. Removing or managing these hosts where practical reduces annual spore pressure.

Resistant cultivars

Per Cornell Cooperative Extension, some plum cultivars show better resistance to black knot than others. American plum cultivars developed at universities in the Midwest and Northeast (Alderman, Superior) tend to show better resistance than European plum cultivars. Consult current regional fruit variety trial data for cultivars rated resistant in your area.

Common problems table

Frequently Asked Questions

How far below a knot do I need to cut?

Per Penn State Extension, at minimum 4 inches below the visible base of the knot, and 6 inches is safer for large or old knots. The fungal mycelium extends significantly into the tissue below the visible swelling; cuts made at the knot surface routinely leave behind infected tissue that produces a new knot.

Will my plum tree recover after extensive black knot pruning?

Per Cornell Cooperative Extension, trees can recover if the crown structure remains after knot removal. A tree where knots are primarily on small lateral and secondary branches can be aggressively pruned and will regenerate. A tree where major scaffold branches or the trunk have multiple knots and require extensive carving may not recover structurally.

Does black knot spread to roses or other plants?

Per Penn State Extension, Apiosporina morbosa infects only Prunus species. It does not infect roses (which are in the same family) or other plants.

How do I identify black knot in early spring before leaves emerge?

Per Clemson HGIC, the black, corky galls are most visible on bare branches in late winter and early spring — this is the ideal time to survey and prune. Walk the tree systematically by branch, checking every scaffold limb. Old knots are unmistakable; look also for the olive-green swellings of first-year knots on previous season's shoot growth.

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Sources

1. Penn State Extension — Black Knot of Stone Fruits
2. Clemson HGIC — Black Knot of Plums and Cherries
3. Cornell Cooperative Extension — Stone Fruit Disease Management