The cause is a fungus named Taphrina deformans . As is often true, the Latin species name describes what happens to the leaves. They become thickened, turn yellow and reddish, and appear deformed. As the disease progresses, a dusty white coating of fungal spores erupts. Severely affected leaves will fall off.
Twigs can show distortion.
Fruit may develop a reddish appearance. Some leaves may remain all summer showing symptoms. The effect on the tree can be profound. Since the fruit's growth depends on active leaf photosynthesis to bring in sugars for swelling and ripening peaches, losing leaves affects the crop and the tree's health. If not controlled, a severe infection can kill a tree in 2 to 3 years.