Ornamental plum trees belong to the Rosacea family and the same genus as cherry and peach trees. The flowering plums sometimes suffer the same diseases as the fruiting varieties. For instance, mealy plum aphids infest and multiply on the fruiting varieties, and then the second- or third-generation aphids migrate to the ornamentals, according to the Washington State University Tree Fruit Research and Extension Center. Fungal infections remain one of the most common ornamental plum tree problems. Fungi stay dormant in winter, then spread through spores during the wet, windy weather of spring.
Ornamental plum trees infected with black knot fungus develop galls, or wart-like growths, on the twigs, branches and trunk. The fungus occurs throughout the United States, but it is more commonly found in the Northeast, according to the Cornell Plant Clinic’s Black Knot Fact Sheet. The disease goes unnoticed until the winter of the second season of infection, when the leaves fall away and expose the galls. Black knot’s extra cellular growth disfigures trees. Over time, the tree weakens and declines. To control the disease, home gardeners must remove and destroy infected twigs and branches, and prune the tree in late winter before the spores release.
Flower parts infected with brown rot turn light brown and often look waterlogged. The fungal disease affects ornamental plum trees in wet climates, particularly the Pacific Northwest. The fungi Monilinia fructicola and Monilinia laxa develop buff-color and grey spores that spread to floral tissue in spring. The fungus enters the pedicil (flower stalk), then the twigs and spurs. The flower collapses and sometimes sticks to the twigs throughout the winter. In severe cases, the twigs die back, which makes the branches and crown appear thin. Prevent brown rot by planting the tree at a site that receives enough sun to dry out wet foliage.
American Plum Borer
Trees infested with the American Plum Borer secrete sap or frass (insect excrement) near the base of the trunk. The pest plagues trees throughout the United States, and the frass is often the only visible sign of damage. Most damage occurs beneath the bark, where moth larvae feed off the cambium, the layer of cells between the bark and the wood. Larvae enter the cambium through open wounds and cracks in the bark caused by disease, landscape equipment and sunscald. The larvae feed horizontally and remove the cambium layers from the trunk. American Plum Borer targets trees suffering from black knot, drought and transplant stress. Michigan State University of Extension recommends a close inspection of the trunk for sap and frass. Place wing traps baited with synthetic female pheromone in the trees, and apply insecticide if the trap catches more than six male adults in a week.
Sunscald causes cracked, dried and elongated areas of dead bark on young ornamental trees. Sunscald damage occurs in late winter when the sun remains in the sky long enough to heat the bark and spur cambial activity within the bark’s cells. As the sun sets and temperatures drop, the warmed cells die. Eventually, the dead bark sloughs off the tree. The thin bark of newly planted trees tends to be more susceptible than the thick bark of established trees. Prevent sunscald damage by reducing temperature fluctuations. Wrap the trunk with a commercial tree wrap designed to reflect sunlight.