Peach leaf curl is caused by the fungus Taphrina deformans, which overwinters in bark and bud scales of peach and nectarine trees. Fungal spores infect leaves and shoots in the spring while leaves are still in the bud and as they just begin to emerge. Mild (50-70°F), wet weather during this period favors infection. Additional spores form on the surface of diseased tissue, and these spores cause new infections if the weather remains mild and wet. Consider applying a single fungicide spray in the fall after leaf drop or in the spring before buds begin to swell to control peach leaf curl (and also plum pockets). Effective fungicide active ingredients include chlorothalonil, copper (e.g., Bordeaux mixture), and ferbam. Choose a fungicide that is labeled for use on edible fruit crops, and read and follow all label instructions to ensure that you use the product in the safest and most effective manner possible.

The UNH extension also mentions: Copper soap (copper octanoate) is effective for peach leaf
curl.