Hello,

The most common diseases on peach in Michigan are brown rot, bacterial spot, constriction canker, and peach leaf curl. Cultural management practices like removing diseased fruits and branches go a long way to reduce future infections.

Copper sprays (usually for peach leaf curl) are usually done at about 75% leaf drop in the fall or before the first bud swell in the spring. Spraying copper on foliage can damage the leaves. 

MSU doesn't have a current guide specifically for home peach production. Some older materials are available on Extension Specialist Bill Shane's share site: https://williamshane.weebly.com/peach.html

Here is a home fruit spray guide from the University of New Hampshire Extension on home fruit trees, which has information about mixing fungicides and insecticides and suggestions for specific targets depending on the time of year.

If you are a home grower you should consider enrolling in MSU Extension's Consumer Horticulture Backyard Fruit 101 course, linked here. They also have articles and other information linked here.

Commercial growers would consult E-154, linked here, and larger scale home growers would follow regional updates from MSU Extension Fruit & Nuts, linked here. The E-154 fruit management guide has estimated dates, but also degree days, which you can track using Enviroweather.