It’s common to see “tar spots” at this time of year, just as autumn is beginning to make its presence felt. Tar spots are the black blotches that you often see on sycamore leaves, as well as on various maples.
The spots are caused by the fungus Rhytisma acerinum, but they don’t do any harm.
The fungus will overwinter in the leaves on the ground beneath the trees and then send out spores in the spring to re-infect a fresh batch of new leaves. Sweeping up and burning leaves might reduce that re-infection rate, but it’s unlikely you’ll ever completely be tar-spot free once it’s in your area.
Traditionally it’s always been believed that tar-spots are an indication of clean air, but that’s more anecdotal than scientifically proven.