Flowering plum trees have pink or white flowers, and leaves that turn bronze or purple in the fall. These trees are usually grown as ornamental trees, but some varieties also bear fruit. Flowering plum trees are low maintenance trees, resistant to many diseases and pests. They are, however, susceptible to several fungal diseases.
The black knot fungus is easily identifiable by the long, hard, black swellings or knots that develop on small branches and, occasionally, on the main branches and trunk. Some knots can be more than 1 foot long. Knots can grow for several years, eventually encircling the entire branch and killing it. When the knots are 1 or 2 years old, they produce spores that are released when the tree blossoms, to infect new shoots. Control black knot by removing and destroying the knots before buds form in the spring. Fungicides can also be used.
The brown rot fungus attacks the flowers, shoots and fruit of flowering plums. Infected blossoms turn brown and remain on the branch. The fungus can also forms cankers at the base of the flower cluster. Green fruits develop small, light brown spots. Ripe fruits can rot in just a few hours and may fall off the tree or remain on the branch. Pale brown masses of fungus spores can completely cover the rotting fruit during wet weather. Control brown rot by removing all cankered twigs and diseased fruit from the tree and the surrounding ground before trees blossom. Multiple applications of fungicide sprays may also be needed.
The plum pockets fungus causes fruits to grow eight to 10 times their normal size and become hollow. These fruits, or "pockets," can be greenish-yellow or brilliant red. Other symptoms include deformed, enlarged shoots, curled leaves and twig death. Control plum pockets by preventing trees from becoming infected in the spring. Use a dormant season spray of lime sulfur, just before the buds start to swell. Lime sulfur can also be used in the fall, after the leaves drop.
Symptoms of shot hole disease include the appearance of reddish or purplish brown spots on new buds in the spring. Spots can also appear later in the season, on leaves, twigs and fruit. The spots grow larger and turn brown, often falling out of the leaves. To control shot hole disease, avoid getting water on the leaves. Remove and destroy infected plant parts as soon as symptoms appear. Shot hole fungus overwinters inside infected buds that look as if they've been varnished. Check twigs for infected buds in the fall, after the leaves fall, and destroy them. In severe cases, a protecting fungicide may be needed to prevent the disease from spreading.
Flowering plum trees are susceptible to cankers that cause dead areas of bark to appear on the branches. The sapwood beneath the bark looks brown, reddish-brown or black. Sometimes cankers will "bleed." When cankers girdle a tree, they keep water from moving through it, causing the leaves to wilt and die. Some cankers cause branch dieback, but cytospora cankers can cause flowering plums to die quickly. Cankers are difficult to get rid of, once a tree is infected with them. Remove cankers by pruning 2 to 3 inches below the canker and minimize tree stress.
- Kansas State University Extension; Canker Diseases of Trees; Ned A. Tisserat
- North Dakota University Extension Hortiscope; Questions On Plum; Ron Smith
- North Dakota State University; Disease Control in Cherries, Plums, and Other Stone Fruits; H. Arthur Lamey; July 1991
- University of California; Preventing Shothole Disease; Gary W. Hickman; November 2001
- Photo Credit Jupiterimages/Photos.com/Getty Images
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