Hypoxylon cercidicola  

Conidial structures developing as a broad palisade on immature stromata, dark reddish brown (macroscopically), often with an olivaceous tinge, deeply wrinkled. Conidiophores not well differentiated. Conidiogenous cells 10-15 x 3-4 µm, ± hyaline to pale brown, proliferating sympodially with small but distinct denticles. Conidia 6-7 x 4-5 µm, obovoid to ellipsoidal, the base slightly apiculate, pale brown to olivaceous, smooth, without an epispore or gelatinous sheath.
Stromata discoid, erumpent from bark, 3-15 mm diam x 1-1.5 mm thick, conspicuously encircled with a rounded margin of fungal tissue covering the ruptured and turned down cortical periderm, sometimes coalescent, usually wrinkled between inconspicuous to conspicuous perithecial mounds; surface dark red-brown to sepia, with orange to orange-brown granules beneath the surface (KOH+ orange to sienna); the tissue below the perithecial layer variable in development, sometimes to 700 µm thick, dark brown. Ascomata perithecia, 250-500 µm diam. and 650-700 µm high, ovoid or infrequently tubular, the ostioles sunken, sometimes surrounded by a ring of paler tissue 90-120 µm diam. Asci 140-175 x 8-9 µm, the spore-bearing part 60-93 µm long, without an apical ring, not blueing in iodine, 8-spored. Ascospores 9.5-11.5 x 4.8-6.2 µm, brown to dark brown, ellipsoidal but strongly inaequilateral and often broadly crescent-shaped,  aseptate, smooth, with a faint straight germ slit the length of the spore on the convex side, with an epispore that is dehiscent in 10% KOH.

Description largely adapted from http://pyrenomycetes.free.fr/