It is caused by Taphrina pruni. This fungus occurs on wild or abandoned plum trees and is rare.
First signs of the disease on fruit are small, white blisters. These enlarge rapidly and soon spread over the entire fruit. The fruit becomes spongy and tissues of the seed cavity wither and die. Fruit become bladder like, abnormally large, and misshapen with thick, spongy flesh. As their spongy interiors dry up, the plums turn velvety gray as spores grow on their surfaces. Infected fruit become hollow in the center, turn brown, wither, and fall from the tree.
New shoots and leaves usually are infected, as well as the fruit. Shoots thicken and often are curled or twisted. Diseased leaves are thickened and curled as in leaf curl.
Spores overwinter on twigs, and during cool, wet periods in early bloom can be splashed to the opening buds, where infection takes place. Developing ascospores give the infected fruit a velvety gray appearance, thus completing the disease cycle.
A spray program similar to the one for peach leaf curl also can control plum pockets.