Pseudopyrenula flavoreagens is characterised by a thallus that lacks a protective outer cortex, presenting a dull and continuous surface that can cover areas up to 2 cm in diameter. Its colour is a whitish grey, distinctly outlined by a black line known as the , approximately 0.2 mm wide, which does not lead to the formation of
galls (abnormal growths) on the host bark. The reproductive structures of the lichen, known as
ascomata, are spherical, ranging in size from 0.25 to 0.35 mm in diameter, and they emerge singly from the thallus. The wall of these structures is , up to 40
μm thick, with the
ostioles (openings) located at the top, remaining separate, flat, and black in colour. Inside, the (the tissue between the spore-producing asci) contains yellow oil globules. Each
ascus holds eight
ascospores, which are clear (
hyaline), segmented into three parts (3-septate), spindle-shaped (), and measure 21–28 by 6–9 μm. The spores have pointed ends, diamond-shaped internal spaces (), and are partly yellow, lacking a gelatinous surrounding layer. , which are another form of reproductive structure, were not observed to occur in this species. From a chemical perspective, the thallus displays a yellow
fluorescence] under
UV light (UV+ yellow).
Thin-layer chromatography analysis reveals the presence of
lichexanthone, a
secondary metabolite in the chemical class known as
xanthones.
Pseudopyrenula flavoreagensis similar in appearance to
P. subgregaria, but the latter species does not have lichexanthone. ==References==