Plum tree rot, more commonly known as black knot, is a serious fungal disease that afflicts plum and tart cherry trees, especially in home orchards where trees are not regularly sprayed and pruned. The fungus produces knots that can cause trees to grow and bloom poorly, producing fewer plums and becoming more susceptible to winter injury. The afflicted tree may eventually die.
Soft green knots start where the leaves attach to twigs. By fall, the elongated knots become brittle, turning brown, then black, spreading on woody portions of twigs and branches. They may spread to main scaffold limbs and even the trunk. Mature knots from several inches to a foot long can encircle an infected twig or branch. A powdery pink or white fungus may cover old knots.
The fungus spends the winter in knots and infected wood. Tiny fruiting bodies on the knots have special sacs that produce spores. In the spring, the sacs send spores into the air when it rains; wind carries the spores to adjoining branches and other trees. Spores that settle on green twigs and current growth and remain wet may germinate, entering breaks in the bark and at the crotches of twigs and branches. Knots may be visible by late summer, but they might not be noticed until the following spring, when they enlarge quickly.
Controlling the Fungus
Avoid planting plum trees next to or downwind from an old or abandoned orchard that has been infected. Remove all wild plum and cherry trees from woodlands or fence rows near your trees; they can potentially harbor the disease.
Check your trees each year before the buds break. Prune 4 to 8 inches from the edge of a knot. If the knot is on a major scaffold limb or the trunk, use a chisel to cut it out; remove at least an inch of healthy wood beyond the edge of the infection. Knots can release spores up to 4 months after they are pruned, so remove them completely and burn or bury them.
The University of Maine recommends a spray of lime sulfur when you prune heavily infected trees that are dormant. Fungicides that have worked against black knot include Captan, copper hydroxide, thiophanate-methyl, lime-sulfur and sulfur. Apply fungicides when the buds break open in the spring and every one to two weeks thereafter, especially before rain. Stop spraying when the terminal growth stops.
Susceptible Varieties of Trees
Ontario Ministry of Agriculture, Food and Rural Affairs says plum tree cultivars especially susceptible to black knot include Bluefre, Damson, Myrobalan B., Ozark Premier, Stanley, Veeblue and Vision. Early Italian, Italian, Valor, Vanier, Verity, Voyageur, Valery, Vanette and V70034 are moderately susceptible. Burbank, Early Golden, German, Shire and V72511 are slightly susceptible.