Leaf curl in peaches and nectarines is caused by a fungus. During the spring when leaves begin to grow, this fungus produces spores which infect the new leaves and young twigs. Infected leaves usually fall from the tree in late spring, however, the twig infections persist, and this is where the fungus survives from year to year.
Therefore, to control leaf curl, your tree must be sprayed in January/early February with a dormant spray. Usually one thorough spray is enough for good disease control. Make sure all twigs, branches, and the trunk are completely covered. If leaf curl disease was severe last season, you should apply a second spray at the time when the leaf buds show a little green tissue (green tip stage). This green tip spray also requires thorough coverage of all parts of the tree.
These dormant sprays need to be an annual cultural practice if you wish to keep your fruit trees relatively free of leaf curl caused by the fungus. Use all plant protection products according to label directions for best results.