COMMON TREE DISEASES
Tar spots are a common problem for maple trees as well as sycamores, box elder, holly and tulip trees. The disease is caused by the rhytisma fungi; most commonly rhytisma acerinum and rhytisma americanum. It is primarily a cosmetic disease that is not fatal to trees. However, untreated ornamental trees in the garden and parks begin to look quite poor and unsightly. The spores of the fungus winter in fallen leaves and are spread by wind and rain in the spring. Though it is not lethal, timely and effective treatments will improve the appearance of the affected trees.
Symptoms: Pale yellow spots appear on the leaves in spring and develop into yellow bordered lesions and a black tar-like spot develops within the yellow area leaving a yellow ring around the black spot.
More information: University of Wisconsin Madison Extension, Cornell University College of Agriculture and Life Sciences