There are several fungi in the genus Rhytisma (most commonly Rhytisma acerinum and Rhytisma punctatum) that cause tar spot on maples and sycamores. These fungi commonly survive in over-wintered leaf litter, where they produce spores that lead to leaf infections.
The first symptoms of the fungus usually start in mid-June as small (less than 1/8 inch diameter), pale yellow spots. The spots enlarge and their yellow color deepens as the season goes on. Next a black spot will develop in each yellow spot by mid-July to early August. The black spot grows in diameter and thickness and soon looks like a spot of tar by the end of summer.
This fungus is only unsightly and NOT detrimental to the tree. Treatment is usually not necessary, costly if attempted and most of the time, ineffective.
The best defense in keeping tar spot out of your trees is to rake up and destroy all infected leaves in the fall. Leaves should be burned or properly mulched. The fungus can overwinter on fallen leaves and provide a source of inoculum to re-infect the trees for the next growing season.
Copyright – Ilex Farrell