Your peaches probably have a disease we call brown rot of peaches and stone fruits. Brown rot is caused by the fungus Monilinia, and is a problem every year in peaches, but more so in wet weather seasons. If you grow peaches or other stone fruits, you should be on a spray program for insects and diseases. Injury to fruits from insects may make brown rot and other fruit rots worse. Several labeled fungicide products or combination fruit tree disease and insect sprays are available for homeowner use. Please use according to label directions. Timing is crucial. Your first fungicide spray should be applied at mid- bloom, followed by another spray at petal fall. You will need to apply another fungicide spray when fruit are small, about 14-21 days before harvest. If weather remains wet, you may need to spray again 10 days later. Remember, fungicide sprays are preventative, and do not cure existing infections. Therefore, sanitation is very important in controlling brown rot. Pick up and discard fruit that fall. Infected fruit that remain on trees are called "mummies" and should be removed, because they are a source of spores the next spring. Trim back any branches that have died, because there may be twig infections that can also over-winter. Trimming branches also increases air circulation in trees which can help to reduce disease.
Hope your fruit harvest is better next year!