Source: Mike Schomaker, Colorado State Forest Service, Bugwood.org
A number of diseases cause curling of leaves.
First evidence of the disease on fruit are small, white blisters. These enlarge rapidly and soon involve the entire fruit. The fruit becomes spongy and tissues of the seed cavity wither and die. Enlarging rapidly, fruits grow 10 times their normal size, turn reddish, and become so misshapen that they are hardly recognizable. As their spongy interiors dry up, the plums turn velvety gray as spores grow on their surfaces.
Spores overwinter on twigs and during cool, wet periods in early bloom may 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.
Routine fungicide sprays normally control this disease in Pennsylvania.