Updated October 2026: corrected the identification features (this species has isidia and a pale tan underside) and the spot tests.
Overview
Gray to blue-gray with distinctive white spots (pseudocyphellae) scattered across its upper surface, Punctelia rudecta is one of the most common foliose lichens on trees in eastern North America. The name 'Punctelia' refers to these punctiform (dot-like) white markings.
It forms loosely attached rosettes up to 15 cm across, with broad, rounded lobes. Tiny cylindrical isidia develop on the upper surface, often from the pseudocyphellae, giving older parts a rough texture in the centre.
Similar to Parmelia sulcata in general appearance but distinguished by the scattered dot-like pseudocyphellae rather than a network pattern.
Identification
- Gray to blue-gray with scattered white dots (pseudocyphellae) on the upper surface.
- Isidia (tiny, cortex-covered cylindrical outgrowths) develop on the upper surface, not powdery soredia.
- Broad, rounded lobes (3–8 mm) forming loosely attached rosettes.
- Underside pale tan to brown with pale rhizines (not black).
- Medulla C+ red, KC+ red (lecanoric acid); cortex K+ yellow (atranorin).
Ecology & Habitat
Common on mature deciduous trees in open woodland, parks, along roadsides, and in suburban settings. Moderately pollution-tolerant and has been expanding its range as air quality improves. Often found alongside Flavoparmelia caperata and Physcia species.
Fun Facts
The scattered white dots on its surface are not damage or disease — they are pseudocyphellae, tiny pores that allow gas exchange through the otherwise waterproof upper cortex.
It is one of the most commonly submitted lichens for identification on citizen science platforms like iNaturalist, thanks to its size, abundance, and photogenic appearance.
Its closest look-alike in eastern North America, Canoparmelia caroliniana, is also isidiate but lacks the white pseudocyphellae and has a dark underside.
Its C+ red reaction produces one of the most vivid colour changes in field chemistry, turning bright crimson-magenta with a single drop of household bleach.
Distribution
Eastern North America, also found in East Asia and parts of Europe