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.