Ornamental flowering plum trees (Prunus spp.) may bear occasional fruit, but they are grown for their aesthetic appeal. In early spring bare tree limbs are awash with white, pink or purple flowers, many of which perfume the air. Their resplendent beauty may be marred by a host of diseases common to all plum species.
Purple-leaf ornamental plum trees (Prunus cerasifera) offer beautiful foliage in addition to their colorful flowers. When winter’s chill is still in the air, “Krauter Vesuvius” bursts into bloom as early as January in temperate climates. Friends of the Urban Forest cites this cultivar is one of the first flowering trees in spring, with fragrant blossoms opening on bare limbs before the purple leaves break bud. “Thundercloud” and “Mt. St. Helens” are other ornamental plum trees cultivars that maintain purple leaves throughout the growing season. The trees grow best in U.S. Department of Agriculture plant hardiness zones 5 through 8.
Cankers, galls and mildews are common fungal diseases of ornamental plum trees. Kansas State University reports that purple-leaf plum is particularly prone to develop Cytospora cankers, which are caused by Leucostoma and Valsa fungi. Cankers injure woody plant parts, prompting trees to ooze amber-colored gum. Black knot galls, which grow in response to the Apiosporina morbosa fungal pathogen, are disfiguring growths on trees. This disease can progress to kill the tree. Powdery mildew affects plum foliage by leaving residue resembling baby powder on leaf surfaces. The responsible pathogen, Podosphaera oxyacanthae, persists throughout winter in leaf litter and on buds. Although chemical controls are ineffective against cankers, some fungicidal sprays with lime sulfur or Bordeaux mixtures may manage black knot, if infected branches are also removed. To manage mildew, use a fungicide that is approved for plum trees.
Bacterial leaf spots and shothole disease, caused by Xanthomonas and Pseudomonas bacteria, are similar in appearance. Initially, spots pepper foliage and as they enlarge, the centers of spots “melt out,” which leaves holes in the leaves. Multiple holes in individual leaves cause foliage to turn yellow before it eventually drops from trees. A soil-borne bacteria, Agrobacterium tumefaciens, infects a tree’s roots or the crown, which is the area just below or just above soil level. Infected plant tissue multiplies to produce a large tumor, called a gall. Bacterial infections are difficult to control chemically, although bactericides may offer some management. Too much nitrogen fertilizer promotes this disease, so avoid areas where plum trees are growing when fertilizing your lawn with a high-nitrogen product.
Many viruses may infect ornamental plum trees. Prunus ringspot virus is the most prolific, attacking nearly all plum species. It causes a wide range of foliar symptoms, with leaf patterns that include spots, rings, bands and mosaics. Advanced disease stages may stunt tree growth, although viruses rarely cause tree death. As with viruses in other plants, there is no cure for these diseases. The best defense is maintaining healthy trees by not over-fertilizing, providing enough water and planting trees in well-draining sites.
- Friends of the Urban Forest: Prunus Cerasifera “Krauter Vesuvius”
- University of Nebraska-Lincoln Extension: Yard and Garden April 16, 2011
- Kansas State University Agricultural Experiment Station and Cooperative Extension Service: Canker Diseases of Trees
- University of Minnesota Extension: Black Knot of Plums, Cherries, and Other Prunus Sp.
- North Dakota State University: Disease Control in Cherries, Plums, and Other Stone Fruits
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