Pertusaria amara (images of British lichens)

Thallus grey, somewhat zoned at the margin, surface with usually abundant, rounded or coalescing, white soralia with a bitter taste (compare with P. albescens); apothecia rare. Widespread and common on bark of deciduous trees.

Refs: Smith et al. (2009), 677; Purvis et al. (1992), 450; Dobson (2005), 322 (photo); Dobson (2011), 323 (photo); Jahns (1983), 254-255 (photo); Whelan (2011), 127 (photo); van Haluwyn et al. (2009), 162-3 (photo); van Herk & Aptroot (2004), 286-7 (photo); Wirth (1995), 2: 696, 699 (photo); Wirth et al. (2004), 299 (photo); Frahm et al. (2010), 85 (photo); Moberg & Holmåson (1984), 208 (photo); Holien & Tønsberg (2008), 178 (photo); Brodo et al. (2001), 527 (photo); Walewski (2007), 102 (photo).

Tasting the soralia is, unfortunately, the quickest way to identify this species – moisten a finger, rub it on the soralia, insert finger in mouth (ideally someone else's) and wait for the bitter taste. This may take a minute or so to develop and it is really, really, not a good idea to think you didn't get enough first time and add more.

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Images of British Lichens