You are encountering brown rot, one of the most common and heartbreaking diseases of apricots, cherries, peaches and plums. Caused by the fungus Monilinia fructicola, brown rot can infect blossoms, immature fruit and ripening fruit. A brown rot crisis often develops just as the fruits begin to ripen. Small wounds made by insects or hail become easy entry points for the fungus, which erupts into round brown spore clusters before taking over the whole fruit. Brown rot is most active in warm, rainy weather.
“Remove any shrunken fruit mummies.”
In addition to using preventive sprays of sulfur or approved fungicides, it is crucial to remove any shrunken fruit mummies that fall to the ground or persist on the trees because the fungus overwinters primarily in mummified fruits. Aggressive pruning and thinning of fruits can help, too, because trees that enjoy excellent sun penetration and air circulation have fewer problems with brown rot.
Even if you do everything you can to keep your trees healthy, your crop can be threatened by windblown spores from wild plums, so this disease is an ever-present threat. Two plum varieties, New York bred ‘Seneca’ and ‘AU-Rosa’ from Auburn University, may be less susceptible than other plums, and ‘Elberta’, ‘Glohaven’ and ‘Babygold No. 5’ peaches provide some natural resistance.