Peach trees suffer from a wide variety of insect and disease pests. Commercial growers will begin spraying during bloom to control the disease blossom blight. Since this disease is only an occasional problem, I normally recommend that homeowners wait until after all of the flowers petals have dropped off the tree.
My first recommended spray begins at what is called shuck split or shuck off. This is when the sepal that used to hold the flowers splits or just falls off (picture). A fungicide is recommended at this time to control the disease scab.
In the spring, peaches should be sprayed at least 2 times, about 2 weeks apart with a fungicide. After that scab should no longer be a problem. You might be able to avoid spraying during the rest of the summer until 4 weeks preharvest, when brown rot sprays will be needed (covered in a later post).
Both organic and conventional pesticides are available for scab control. Wettable sulfur has been successfully used for years, although the interval between sprays may need to be shortened to 7 to 10 days. Three sprays instead of 2 will be needed for sulfur. Captan is a good conventional fungicide, but you may have to go to a farm and garden shop to find it. Scab picture by Dave Rosenberger, Cornell University