Borrer's Speckled Shield Lichen

The thallus of Punctelia borreri is foliose (leafy), forming rosettes that are closely attached to their substrate, measuring up to 10 cm (4 in) in diameter. The colour of the upper thallus surface ranges from grey to bluish-grey, greenish-grey, or yellowish-grey. Old herbarium specimens turn buff. Sometimes, the margins of the thallus are whitish from pruina (a powdery coating); at other times they have a brownish tinge. The pruina is often abundant, but not present on all the lobes. In fresh specimens, the pruina gives the lobes a glaucous colour (dull bluish-green). Pruina consists of crystals of calcium oxalate, an insoluble organic calcium salt that is excreted on the thallus surface. The surface has a smooth and shiny texture, with some wrinkling near the centre of the thallus. Lobes comprising the thallus are flat or concave, measuring 4–8 mm (0.2–0.3 in) wide. Pseudocyphellae are small (up to 300 μm wide), roughly circular, and point-like (punctate); they are more prominent towards the periphery of the thallus. Closer to the centre of the thallus they aggregate in regions with powdery propagules called soralia. The soralia are dot-like, whitish, and distinctly rounded. They arise from pseudocyphellae that are small and punctiform, which often become confluent in age. The soredia are farinose (powdery) to more or less granular on older thalli. …

Punctelia borreri usually grows on bark, but is occasionally found growing on rock. The lichen has a cosmopolitan distribution, having been recorded from Africa, Asia, Europe, North America, Oceania, Macaronesia, and South America. Its distribution in North America is largely limited to the west coast from California to Canada. Although some occurrences have been reported in Ohio and West Virginia, it is rare in eastern North America. In Mexico, it only occurs in Veracruz. In Australasia, Punctelia borreri is widely distributed, having been reported throughout eastern Australia and New Zealand. It is fairly common in eastern Australia (including Queensland, New South Wales, and Victoria) but is rare in New Zealand. In India it occurs in subtropical and lower temperate regions at altitudes of 1,500–2,200 m (4,900–7,200 ft). A 2017 record from the Hunza Valley in Pakistan was the first documented occurrence for that country.

Before its range expansion in Europe, Punctelia borreri was noted to have a strongly oceanic distribution with scattered occurrences in the central part of the continent. In a 2008 checklist of all parmelioid lichens in Europe (updated in 2011), P. borreri was noted to have been recorded from 21 countries and island groups. …

Several endophytic fungi (fungi that live within the tissue of plants or lichens) have been isolated from the thalli of Punctelia borreri. These include Chaetomium globosum, Hypoxylon fuscum (conidial stage), Nodulisporium hyalosporum, Spirographa ciliata (conidial stage), as well as taxa not identified to species from the genera Chaetomium, Nodulisporium, Phoma, Thielavia, and Scopulariopsis. Two species of lichenicolous fungi that have been recorded parasitizing Punctelia borreri are Nesolechia oxyspora and Pronectria subimperspicua. Infection by the latter of these fungal parasites creates discoloured areas on the host thallus.

Punctelia borreri has been used in traditional Chinese medicine as an alleged remedy for a variety of ailments, including chronic dermatitis, blurred vision, bleeding from the uterus or from external injuries, and for sores and swelling. To use, a decoction was drunk, or the dried and powdered lichen applied directly to the affected area. Laboratory tests using the disk diffusion test method have shown that ethanol-based extracts of P. borreri have antimicrobial effects against the bacteria Xanthomonas campestris and Staphylococcus aureus. In a study exploring the potential of several lichens as antidiabetic agents, P. borreri was noted for its in vitro ability to inhibit α-amylase, a key enzyme of carbohydrate digestion.