A close-up would be more helpful, but this potentially could be brown rot. Increased humidity and poor sanitation are major factors for this disease to exist. If you start seeing these dark spots, immediately talk them off the tree and discard them (green waste, burn, etc.). It also helps if you open up the canopy a bit with some strategic pruning, which also helps reduce humidity. If it's really severe you may want to apply a copper-based compound to knock back the pathogen population so just cultural controls are effective enough in the future.