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Stigmidium epixanthum Hafellner
Family: Mycosphaerellaceae
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Vegetative hyphae: hyaline, hardly discernible in thallus sections Pseudothecia: black, globose to obpyriform, sessile, 100-140 µm high and 120-180 µm wide wall: consisting of 2 layers, the outer layer dark brown, entirely carbonized, inner layer basally hyaline to pale brown hamathecium: with pseudoparaphyses of type a asci: clavate to saccate, 40-60 x 12-15 µm, 8-spored, with ocular chamber; endoascus: I -and KI -, BCr- ascospores: hyaline, 1-septate, not constricted at the septa, upper cell slightly thicker than lower one, ellipsoid , 11-17 x 5-7 µm; walls: smooth, thin, BCr- Pycnidia: not seen. Hosts: thallus areolae or sometimes on apothecial discs of Acarospora subg. Xanthothallia, incl. A. heufleriana, A. lavicola, A. radicata, A. schleicheri and A. xanthophana World distribution: Africa, Australia, and North and South America Sonoran distribution: known from many localities in Arizona and southern California, Baja California, and Baja California Sur, Chihuahua, Sinaloa and Sonora. Notes: There are at least three species of Stigmidium s. l. occurring on Acarospora: S. epixanthum, S. fuscatae (see below) and S. rouxianum Calat. & Triebel (see Calatayud and Triebel [2003]). The ascomata of S. epixanthum are mostly sessile, are often found scattered or in irregularly small groups, preferring the edges and fissures of the yellow thallus areoles.