QI have scrubby woods at the edge of my property, and I've noticed several small trees with odd growths on the branches. The growths are brownish black, sort of lumpy and look like a bunch of burned marshmallows on a stick. What causes this?
AYour description sounds like black knot disease. Black knot is a fungal disease that affects several cherry species and some plums (both cherries and plums are in the genus Prunus). Black knot is a particular problem on common chokecherry (Prunus virginiana), a small, shrubby tree that is native to a wide area from North Carolina to Kansas and north to Saskatchewan and Newfoundland. This tree is commonly found in wooded or semiopen areas in this region, so it's probably the species you are seeing. Unfortunately, black knot also affects 'Canada Red Select,' the popular purple-leafed ornamental form of common chokecherry.
The fungus that causes black knot initially infects stems of chokecherry and other susceptible species. The first sign of infection is some swelling and possibly small cracks on the branches. The following spring, obvious swollen knots form on the stems. These swellings develop the typical black, charred look later in the growing season. The black knots are often unnoticed until leaves fall in the autumn, so it's not surprising that you have just noticed them now. If these black knots are not removed, they will release fungal spores in the spring, which can then infect more trees. Untreated, black knot causes branch loss and eventually can kill the plant.
Unless you or neighbors have susceptible Prunus species in your yards, there is no reason to try to control the black knot on chokecherries growing wild. If the disease is affecting your ornamentals, however, you may need to control the black knot. In mid-to late winter, prune all infected branches to a point at least 3 to 4 inches below the knots. Dispose of the pruned branches by burning them, where it's legal, or throwing them in the trash. If infected branches are left on the ground, the spores will continue to spread. If possible, remove any wild infected plants. Look for new swellings on branches during the spring and summer, and prune infected branches and dispose of them. Spraying plants with lime sulfur in early spring, before leaves emerge, may also help prevent black knot, but spraying is only recommended for severe infestations on valuable ornamentals.
-- Nancy Rose is a research horticulturist at the Minnesota Landscape Arboretum. She spends her spare time gardening, inside or outside, depending on the weather. To ask her a gardening question, call 612-673-9073 and leave a message. She will answer questions in this column only.