In these last few weeks before flowers and leaves cover the stems of your cherry and plum trees, inspect them carefully for signs of black knot. The main symptom of this disease, which is caused by the fungus Dibotryon morbosum, is a hard, warty, black gall. Though the disease is most common on cherry and plum trees, it also infects chokecherries, peaches, and apricots. It can be present on both wild and cultivated varieties.
Damage
The black knots for which the disease is named weaken trees by interfering with the flow of nutrients and water inside branches. Twigs die quickly; larger branches can last for years. Trees with many knots can become stunted and insects can invade the tree through older knots. The stress on the tree may render it susceptible to other infections. The fungus can easily spread to infect neighboring Prunus trees.
Control
By the time you see the black warty knots on your tree’s branches, the tree has already been infected for a long time—generally a year or more—but in most cases you can still save the tree. In late winter (before new growth appears), prune out all black knots. Make the cut at least 3 to 4 inches below the knot to get rid of all the fungus. Between cuts, dip tools in a 10 percent bleach solution. Also look for swellings or cracks in the outer bark, as this is how the infection looks when it first appears on the tree. (If you’re not sure, watch the cracks during springtime: if a velvety green growth covers the area, it’s a black knot infection in its first season.)
If you live near wild plums or cherries, they can be a source of the black knot fungus: if it’s possible, check them and remove any black knots, or destroy the tree if the infestation is too serious to treat. Fungicide is generally not recommended except in extreme cases.
—Photo courtesy of the Kemper Center for Home Gardening at the Missouri Botanical Garden.