Black Knot

Hard black swellings on a plum or cherry branch are black knot. Cut at least four inches below each knot in late winter, while the wood is easy to see, then burn, bury, or bag the prunings, because knots left under the tree still throw spores in spring rain. A spray will not shrink a knot that is already there.

People still say Dibotryon. Then the real name: black knot, Apiosporina morbosa. It is a fungus of Prunus wood, not a fruit rot and not the weed Japanese knotweed. Young knots are olive-green and corky the first summer. By the second year they harden into rough black spindles that can run from a half-inch to a foot or more and may bend the twig to one side. They show first on plum, prune, and chokecherry, and they also sit on many cherries, including purple-leaf plum and wild black cherry at the woods edge. The fungus lives in the knot and in a few inches of wood past the visible swelling, so a cut that only nicks the gall leaves the infection in the branch.

Cut four inches below the knot, then get the wood off the property

The finish is a saw, not a bottle. Walk the tree in late winter, February or March in much of the North, when the leaves are off and the knots stand out against bare wood. Cut at least four inches below the lowest part of the knot, and six to eight if the limb can spare it, because the fungus grows past the black swelling. On a trunk or a major scaffold you cannot remove, chisel the knot plus about an inch of healthy bark, knowing that wound can later invite bacterial canker. Wipe the blades with 70 percent rubbing alcohol, or a 10 percent bleach mix, between cuts, then rinse and oil if you used bleach so the steel does not rust.

Get the wood off the property. Burn it where the town allows, bury it, or bag it for trash, because knots still throw spores after they are off the tree. A pile under the plum is still a source. Nearby wild plum and chokecherry with the same black swellings keep feeding your tree, so prune those knots too if you can reach them, or take out a volunteer you do not need. Inspect the same tree again next winter, because swellings you cannot see this year will blacken the year after infection.

1. Confirm. After leaf drop, a hard, elongated, rough black swelling glued into plum or cherry wood is this fungus. An olive, velvety edge in early summer is the same disease, one year younger.
2. Cut at least four inches below the knot in late winter, farther if the limb can spare it. Chisel a trunk knot plus about an inch of healthy bark. Disinfect the blades between cuts.
3. Remove the wood from the yard. Burn, bury, or bag it. Do not leave it under the tree, and do not drop it in a cold backyard compost heap.
4. Walk it again next winter. New black spindles are last spring's infections showing late, not proof the first prune failed.

A spray will not shrink a knot that is already black

A labeled protectant can shield this year's green shoots after you have already cut the knots out. It does not melt a gall that is already hard. Begin when flower buds start to open, and only on a plant named on the label, because edible plum and ornamental cherry are not always the same product. Repeat before wet weather until new shoots harden, then stop. Wisconsin landscape advice skips the spray because it is expensive and often fails if the knots stay on the tree. That is the honesty: prune first, and treat a bottle as a spring shield on a young tree you are trying to keep clean, not as the numbered finish.

A few galls on a chokecherry or sour cherry that still pushes normal leaves can wait, because many Prunus trees carry black knot with little health loss. Act when leaves wilt and brown beyond the knot, when a Stanley or purple-leaf plum is loading up, or when the knots sit on the trunk and major scaffolds. Pulling those limbs would destroy the tree's shape, so removal is cleaner than a skeleton. Managing an orchard that already shows knots is a two-year job, because this year's infections stay invisible until next season. Cracked, oozing galls on a large trunk do not mean black knot itself is rotting the wood, but wood-decay fungi can enter those cracks, so have an arborist check whether the tree is still stable. Japanese plums often take it lighter than American and European types. That is a buying note, not a cultivar catalog.

Hard black swellings on plum and cherry wood

Look on twigs, fruit spurs, and branch crotches after leaf drop, when the galls are easiest to see. A young knot is a light-brown or olive swelling that can feel corky and, in early summer, wear a velvety green coat of spores. By the next fall it is hard, brittle, rough, and black, often spindle-shaped, and it may wrap the twig. Small twigs often die within a year. Larger limbs can live several years before the gall girdles them. Old knots may go white or pink when other molds move in, so a pale crust on a black spindle is still this disease, not a different one. Peach and apricot can get it, but they rarely do in a home yard.

A shepherd's crook is fire blight, not this knot

Fire blight burns a shoot tip and leaves the dead leaves hanging, often with a crook at the end. That is a bacterium, not a woody black spindle glued into last year's growth. See fire blight. Do not run that blossom-cut calendar here.

Crown gall is usually a smoother tumor at the crown or roots, sometimes on a trunk. Black knot is darker and rougher and sits out on the limbs. Soft brown fruit with a gray fuzz, or mummies on a plum, is brown rot. Hard black raisins on a grape cluster are grape black rot, a different fungus on a different plant. A sunken gumming wound without a spindle is bacterial canker. Thick puckered peach leaves are leaf curl. Japanese knotweed is a plant people search because the names share a word. It is not a gall on a plum.

Related reading

• Fire blight — shepherd's crook and hanging leaves, not a black spindle on the wood.
• Brown rot — fruit rot and blossom blight on stone fruit, not this branch gall.
• Black rot — grape mummies on the cluster. The name is a leftover, not this Prunus fungus.
• Botryosphaeria canker and dieback — sunken canker and wilt, not a rough black knot.
• How to grow Prunus — plum and cherry care. Keep the tree vigorous so a light load of knots is easier to wait out, and inspect nursery stock before you plant.