Black Knot of Plum
Black knot of plum, caused by the fungus Apiosporina mobosa, is well-named because of the characteristic black, warty knots it forms on the branches of infected trees. Infected trees grow poorly and gradually become stunted; occasionally, their limbs are girdled. The disease is most important on plum, prune, and sour and wild cherry trees.
The disease is present only in the woody parts of trees, occurring most frequently on twigs and branches and sometimes on trunks and scaffold limbs. The warty swellings first become visible in late summer or the following spring on new shoots. At first, the knots are somewhat greenish and corky, but with age they become black and hard. They vary in length from an inch to nearly a foot. Often, they do not completely encircle the branch. Those a year old or older can become covered with the pinkish-white mold of another fungus and become riddled with insects, especially the lesser peach borer. Limbs and sometimes whole trees are stunted and eventually killed by the ever-expanding knots.
Around the time new seasonal growth is 1/2 inch long, spores of the fungus are discharged from tiny sacs in the surface of the knots. These are spread by rain and wind to the new growth, where infection takes place. Spore discharge and infection are greatest during wet periods, at temperatures ranging from 55 to 75°F. Infections continue to occur until terminal growth stops. A few greenish, corky swellings might become visible the fall after infection occurs, but most will not be noticed until the following spring. Generally, the knots produce no spores until the second spring after they become visible. The fungus in woody tissues continues to grow in the spring and fall, increasing the length of the knots. Their eventual size depends greatly on the host species and variety.
New plantings of plums should not be made next to old ones with black knot. Remove any wild plum and cherry trees from nearby woods and fence rows for at least 500 feet from the new orchard. Inspect orchards and surrounding wooded areas each winter for knots and prune out infected shoots and limbs. Once the disease appears in the trees, remove and burn the knots before bud break. When they occur on twigs and small branches, prune out the infected branches about 4 inches below the knot. Cut out the knots on large branches and trunks. This is done most successfully during August when the fungus does not extend far beyond the visible swelling. Remove the diseased wood and about 1 inch of clean wood around the knot. It is best to prune off the knots before growth begins in the spring. Remove the prunings from the orchard, as they will continue to produce spores for several weeks after removal. Plant black knot-resistant varieties when possible. The plum varieties Shropshire and Stanley are highly susceptible; Brodshaw, Early Italian, Fellenburg, Methley, and Milton are moderately susceptible; Formosa, Santa Rosa, and Shiro are slightly susceptible; and President apparently is resistant to black knot.