Peach leaf curl is caused by the fungus Taphrina deformans. The fungus overwinters on twigs and bud-scales. Spores are washed into buds soon after the buds begin to swell in spring. The spores infect the developing leaf and bud tissue, and the fungus then grows with the expanding leaves and buds. The first symptoms are usually noticed when trees reach the Petal Fall stage.
Affected blossoms are also misshapen and frequently fail to set fruit.
Most affected leaves drop from the tree after petal fall. The new growth that develops after petal fall is not affected and quickly hides the defoliation caused by leaf curl. A few scattered infections cause no significant damage to the trees, but they can provide overwintering inoculum for the following year. Three factors probably contributed to development of peach leaf curl during the past several years. First, most peach trees in NY had no crop in 1994 because of cold winter temperatures.