Rhytisma acerinum

Hosts: Acer platanoides, campestre [the authors suggest that the similar fungus on Acer pseudoplatanus reported elsewhere as R acerinum may be a distinct species but do not evaluate that themselves]

The fungus on Acer platanoides seems to be limited only to that host species. In many instances where heavily infected A platanoides grow next to native (to North America) red, silver, and sugar (A saccharum) maples, the other host species remain free of disease. Second, the morphology of the fungus on A platanoides differs from that on A rubrum or saccharinum. The former [R acerinum] matures in autumn as relatively large (20+ mm diam) black spots. [It] produces copious amounts of conidia from mid-August until leaf fall in late September.

First symptoms of impending disease are chlorotic [yellow] spots that appear on new foliage in central NY state from mid-June to the first week in July. Within 3 weeks, each chlorotic spot usually holds a dozen or more discrete black masses of fungus tissue each about 1 mm diameter and vaguely reminiscent of spots caused by R punctatum on various maples. Stromatal tissue continues to enlarge (faster on shade leaves than in sun) until the individual masses in each spot coalesce to form a stroma averaging 13 mm in diameter. Some spots are more than 25 mm wide. Furthermore, the stromata [of R acerinum] are much thinner than those [of R americanum], rarely exceeding 0.5 mm. By mid-August, from 6 to 18 individual pycnidia, each discernible by a minute papilla at its ostiole, can be seen from above. As the time of normal leaf drop approaches, developing apothecia also become visible as discrete blisters of bumps in the rind. These rarely coalesce and never assume the sinuous character of [R americanum].

A tar spot resembling the A platanoides pathogen is occasionally found on introduced specimens of a European native, Acer campestre. [genetic evidence is consistent with a monophyly of Rhytisma on Acer campestre and platanoides but doesn't confirm they're the same species]

- GW Hudler, S Jensen-Tracy, MT Banik: (1998) Rhytisma americanum sp.nov.: a previously undescribed species of Rhytisma on maples (Acer spp.)^©

Reference: http://www.cybertruffle.org.uk/cyberliber/59575/0068/0405.htm