It is caused by the fungus, Taphrina pruni . This fungus occurs wild or on abandoned plum trees and its occurrence is rare. Symptoms First signs of the disease on fruit are small, white blisters. These enlarge rapidly and soon cover 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 are usually infected as well as fruit. Shoots thicken and are often curled or twisted. Diseased leaves are thickened and curled as in leaf curl.