Rhytisma acerinum

Rhytisma americanum

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]

Rhytisma americanum, sp. nov.

Hosts: Acer rubrum, saccharinum, saccharum [later work by Hsiang et al suggests the Rhytisma on Acer saccharum is more likely R acerinum]

[R americanum] matures as circular masses of stromatic tissue rarely more than 10 mm in diameter. 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. In the case of [R americanum], a small (<0.5 mm diam) black mass of fungal tissue [pycnidia] appears near the center of each spot. Pycnidia are detectable for no more than 3 weeks; thereafter the spots rapidly grow to become typical thickened black stromata comprised of fertile white fungus tissue overlain with a black rind. By autumn, from 2 to 12 parallel sinuous channels of ascogenous tissue are discernible by the depressions they case in the rind of each stroma. In many cases stromata are surrounded by a bright yellow border of chlorotic host tissue.

Stromata: on abaxial surfaces only of host leaves, black, more or less cylindrical except where limited by major veins, diam variable but rarely less than 3 mm or more than 10 mm, up to 1.5 mm thick at center and tapered to leaf surface at perimeter, occasionally surrounded by a bright yellow halo of chlorotic host tissue. In surface view: ascomata develop as parallel sinuous channels, extending up to the entire length of the stroma.

Habitat: on leaves of Acer rubrum, A saccharinum, and rarely, Acer saccharum, Common in North American east of the Great Plains, uncommon elsewhere.

Rhytisma americanum sp.nov.: a previously undescribed species of Rhytisma on maples (Acer spp.)