Lecanora thysanophora
Mapledust lichen
Mapledust lichen is common in eastern North America and many other parts of the world.
Identification: This lichen is pale yellow to greenish. It is thin and powdery looking, and may appear as a continuous coating, or be divided into patches. It is often scaly (leprose). A white fibrous band, the prothallus, borders the lichen. On close inspection, there may be one or two bluish zones of hyphae, the actively growing regions.
Habitats: On the trunks of deciduous trees, especially Acer saccharum and Thuja occidentalis, as well as occasionally on shaded siliceous rocks.
Close-up with mapledust lichen thallus (greenish, right), white prothallus border, and blue-gray line of hyphae. 
Online References:
References:
On large sugar maple. · 9/26/2010 · Brooks Woodland Preserve, Swift River Tract, Petersham, Massachusetts · ≈ 8 × 5″ (19 × 13 cm) 
ID is uncertain 
On large sugar maple. · 9/26/2010 · Brooks Woodland Preserve, Swift River Tract, Petersham, Massachusetts · ≈ 8 × 5″ (19 × 13 cm) ID is uncertain 
9/28/2011 · Groton Place and Sabine Woods, Groton, Massachusetts · ≈ 1¾ × 2½″ (4.6 × 6.9 cm) ID is uncertain 
5/4/2023 · Tarbox Preserve, Topsham, Maine 
5/4/2023 · Tarbox Preserve, Topsham, Maine 
4/21/2023 · Robert P. Tristram Coffin Wildflower Preserve, Woolwich, Maine ID is uncertain 
10/2/2023 · Marsh View Loop, Ridgewell Preserve, Phippsburg, Maine 
4/10/2010 · Nashua River Rail Trail, Groton, Massachusetts · ≈ 4 × 6″ (10 × 15 cm) ID is uncertain 
9/28/2011 · Groton Place and Sabine Woods, Groton, Massachusetts · ≈ 4 × 6″ (10 × 15 cm) ID is uncertain 
9/28/2011 · Groton Place and Sabine Woods, Groton, Massachusetts · ≈ 3 × 4½″ (7.9 × 11 cm) ID is uncertain 
9/28/2011 · Groton Place and Sabine Woods, Groton, Massachusetts · ≈ 4 × 6″ (10 × 15 cm) ID is uncertain 
| Kingdom | Fungi | Mushrooms, lichens, gel fungi, yeasts, and molds; also called eukaryotes |
| Division | Ascomycota | Sac fungi |
| Subdivision | Pezizomycotina | Sac fungi with filaments |
| Class | Lecanoromycetes | “Lichenized” fungi |
| Order | Lecanorales | Mostly lichen-forming fungi |
| Suborder | Lecanorineae | Lichens |
| Family | Lecanoraceae | Lichenized fungi includes many genera, named after Lecanora |
| Genus | Lecanora | Means "having a light-colored margin of apothecium formed by thallus" (from Stearn's Botanical Latin) |
| Species | thysanophora |
Lecanora thysanophora description by Thomas H. Kent, not updated.
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