Late dormant- what does that mean? Were trees starting to grow?
In spite of a cool wet spring, I got excellent results at 3 sites that have been hammered in the past- and on a lot of trees at each site (5-20). I put the copper spray down right before first growth- no green showing yet but showing about a week later. This year I used Kocide.
A single application in Spring has consistently provided results of disease reduction to the point of having only a couple of most susceptible varieties with a few damaged leaves- not enough to matter, even for these.
Trees should be pruned wide open with underside shoots removed from the tree. Shaded leaves are most susceptible (invasion points) as are entire trees blocked from eastern sun, which delays the evaporation of morning dew.
Peaches in my region are one of the better species to grow in relatively shady sites, but PLC is one issue to be concerned about.