Black knot

Apiosporina morbosa

Hard black warty swellings that girdle plum and cherry branches like a lump of coal. Cut every knot out in winter, twenty centimeters below the swelling.

Fungus Serious Airborne sporesRain splash Spreads to neighbors

What you'll see

A branch swollen into a hard, rough, coal-black knot two to thirty centimeters long, running along the stem rather than around it.
In the first year the swelling is soft, olive-green and easy to miss; the black comes in the second.
Leaves beyond a knot wilting and the shoot dying back, because the knot has girdled it.
Knots on the trunk or a main scaffold, which eventually kills the whole limb.
Old knots cracked, whitened by a secondary mold, and no longer producing spores.

Why it happens

A fungus of the plum and cherry family. Spores are released from the second-year knots in wet weather each spring, exactly as the new shoots are extending, and infect that soft green growth. The infection sits invisibly through the first summer, swells the wood the following year, and starts producing its own spores the year after — a three-year clock that is why the problem always seems to arrive from nowhere.

Wet springs and infected wild plum or cherry nearby are the two things that matter. Common throughout eastern and central North America; ornamental and fruiting plums, and Canada red chokecherry, are the classic victims.

Treating it

Prune every knot out in late winter while the tree is dormant and the fungus is not releasing spores.
Cut at least 20 cm below the visible swelling — the fungus is in the wood well beyond what you can see.
Burn or bin the prunings, and do not leave them on the ground; a cut knot still releases spores for months.
Where a knot is on the trunk and cannot be removed, cut the wood away to 2 cm outside the swelling, or accept that the tree has a limited life.
Disinfect the saw between cuts with alcohol.

Keeping it away

Remove wild plum, chokecherry and neglected ornamental plums within a hundred meters if you possibly can.
Inspect the whole tree every winter; it takes ten minutes and catches the knots while they are small.
Plant resistant cultivars — President and Damson plums hold up far better than Stanley.
Avoid heavy summer pruning, which pushes soft new growth into the infection window.
Protect a young valued tree with a fungicide at bud break in a wet spring, alongside the pruning rather than instead of it.

Who gets it

Plums and cherries, wild and cultivated: damson, Japanese and European plums, chokecherry, sour cherry and ornamental Prunus.