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DescriptionBasidiomata: rarely present, white to pale yellow, pellicular; hyphae: hyaline, cylindrical, thin- to thick-walled, without or with scattered clamps; basidia: usually with 2 sterigmata, without basal clamps, 20-33 x 5-8 µm; sterigmata: 5-6 x 1.5-2 µm; basidiospores: narrowly ellipsoid, 8-11 x 4-5.5 µm; Anamorph: forming a whitish layer over carrots in cold-storage; sclerotia: relatively common, brownish, ellipsoid, up to 1 mm.; Hosts: on epiphytic lichens and algae; in the Sonoran area over the thallus of Physcia aipolia; World distribution: Europe, northern Africa (Tunisia), North America; Sonoran distribution: once collected over lichens in central Arizona.; Notes: Athelia arachnoidea is an extremely common pathogen of epiphytic lichen and algae communities, killing the entire corticolous vegetation over larger areas, often invading the entire trunk of a tree. As basidiomata are rarely developed, the presence of the fungus is usually recognized by the abundant white hyphae overgrowing the lichens. Conspicuous sclerotia are frequently present, including during seasons of low hyphal growth, and reveal the presence of the fungus. Non-lichenicolous Athelia-specimens growing directly over wood are often attributed to the same species, but molecular studies proving their conspecificity are still missing. Using non-lichenicolous material, the teleomorph-anamorph connection with Fibularhizoctonia carotae, a cold-storage pathogen of carrots was established by Adams and Kropp (1996). Several other Athelia species have also been found as lichenicolous, but they appear to invade lichens more rarely than A. arachnoidea.