Location: upper leaf, lower leaf, between leaf veins
Form: non-gall, leaf spot
Cells:
Possible Range:
The gall's range is computed from the range of all hosts that the gall occurs on. In some cases we have evidence that the gall does not occur across the full range of the hosts and we will remove these places from the range. For undescribed species we will show the expected range based on hosts plus where the galls have been observed.
Our ID Notes may contain important tips necessary for distinguishing this gall
from similar galls and/or important information about the taxonomic status of
this gall inducer.
Created Feb 4, 2026 1:47 PM UTC
•
Last updated Feb 4, 2026 1:47 PM UTC
Rhytisma americanum sp.nov.: a previously undescribed species of Rhytisma on maples (Acer spp.)
GW Hudler, S Jensen-Tracy, MT Banik
(1998)
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.
