Cecidonia xenophana (Körb.) Triebel & Rambold
Phacographa xenophana Körb.
Lecidea alumnula Nyl.
Thallus whitish forming growths in the form of blisters on the thallus of the host, whitish. Apothecia buried in the thallus at the beginning and later slightly raised, 0.2-0.5 mm in diameter, disc flat, sometimes with a small umbo in the centre, black, margin thick, persistent, black. Spores elliptical, simple, hyaline, 10-13 x 5-7 µm. Photobiont: Trebouxia. Negative colored chemical reactions. Medulla: I-. Rare parasite of Porpidia, in particular of Porpidia irrigua.
N.B. Obligate parasite of Porpidia but can sometimes form thalli, apparently independent, at the end after having destroyed that of the host.