In my neighborhood we just can't grow peaches or nectarines because of Brown Rot disease, caused by the fungus Monilinia fructicola. A much diseased nectarine is shown at the right, and a close-up of the fungus is shown at the left.
The fruit at the right is gradually drying out and, if it had not been picked, would have remained on the tree, slowly dried out, and eventually formed a dark, puckery "mummy," or stroma, which may have eventually fallen off, or remained on the tree during the upcoming winter.
When spring comes, mycelium within the hardened and blackened mummy produce cup-shaped apothecia a little like the cup fungus shown on our cup fungus page. That's not surprising, since Brown Rot is an Ascomycota, or Cup Fungus. The cup structure then discharges ascospores that infect young flowers and leaves of peach trees and close relatives to peach trees. During spring and summer, chains of conidia, which are nonsexual spores, appear on infected leaves and young twigs, and these spores then infect other trees.
This is a hard disease to get rid of! Of course, one important way to reduce the chance of getting the disease the next season is to remove fruit mummies and other obviously infected parts of the tree, but my experience is that once you have it you might as well forget about growing peaches, unless you're willing to do a lot of fungicide spraying.