The thundercloud plum tree (Prunus cerasifera ‘Thundercloud’) is a small deciduous tree that produces edible purple fruits in the fall. The fragrant white or pink flowers appear in the spring. The foliage emerges ruby red and turns reddish purple as it matures. Thundercloud plum trees grow in U.S. Department of Agriculture (USDA) hardiness zones 5 to 8 in full sun and well-drained soil. They grow best in acidic soil, but will also grow in alkaline soil, and are moderately drought tolerant. They are prone to several fungal diseases that affect the trunk, foliage and fruits.
Trunk and Branch Diseases
Poor growth and undersized leaves are the symptoms of armillaria crown and root rot on thundercloud plum trees. It is caused by soilborne fungus that produces mats of white growth under the bark of affected trees.
Crown galls are soft spongy wart-like growths on the crown or roots of thundercloud plum trees. As the galls mature, they become rough and hard, and may girdle the trunk and kill the affected tree.
The cytospora fungi forms elongated cankers on the trunks and branches of thundercloud plum trees. It is a perennial canker that continues growing each year and causes discolored foliage and drooping branches.
The foliage of thundercloud plum trees can develop small circular leaf spots. The leaf spots are purplish and turn brown as they get bigger. The centers of the leaf spots may drop out causing shot holes.
Rust causes yellow spots on the upper sides of the leaves, and brown spore masses on the underside of the leaves. The leaves appear yellowish and drop prematurely.
Leaves affected with silverleaf are ashy gray in color with a green tinge and curl slightly. As the disease progresses, the affected branch declines and dies.
Blossom and Fruit Diseases
Brown-end blossom rot affects the blooms on thundercloud plum trees in the spring. Infected blooms turn light brown with buff or gray areas of spores. The disease can spread to small twigs and branches.
Plum pocket disease on thundercloud plum trees causes distorted growth of leaves, limbs and twigs. Infected fruits turn yellow and become swollen and bladder-like.
Russeting causes scabs to appear on the fruits three to four weeks after full bloom. As the disease progresses, the shiny scabby areas form irregular bands, which turn brown and become rough.