Dormant spraying is an effective & relatively safe way to prevent many of the insect & disease problems that plague our gardens. It’s called dormant spraying because it applied to deciduous plants when they are dormant & leafless.
There are two types of dormant sprays. Liquid copper (copper ammonium complex or copper octanoate) is an effective fungicide & bactericide. Horticultual oil smothers overwintering insects & insect eggs. Copper & oil can be combined & applied at the same time.
Peach leaf curl is best controlled with a dormant spray. Unfortunately, the two most effective sprays, tribasic copper sulfate (microcop) & lime sulfur (polysul) are no longer available. Use copper ammonium complex (Monterey Liquicop) with at least 1% horticultural oil. I use 4T Liquicop + 5T Volck oil per gallon of water. I have not found copper octanoate to be effective. I spray 3 times (Thanksgiving, New Years, Valentine’s Day) for trees prone to leaf curl. Trees need to be sprayed before they break dormancy.
http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7426.html (peach leaf curl)
Dormant spraying will also help control scale, brown rot & aphis.
http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7408.html (armored scale)
http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7408.html (soft scale)