Summary:
Features include 1) an orange layer growing over polypores and other fungi, turning violet to purplish red in KOH, 2) orange red perithecia (flasks) forming small pimples on surface, turning purple red in KOH, and 3) ascospores that are two-celled, verrucose, apiculate, and usually less than 25 microns in length. The associated anamorph is Cladobotrum varium Nees.

Microscopic:
SPORES (13)20-25(27) x (3)4-6(7.5) microns, fusiform to lanceolate, verrucose (verrucae <=0.5 microns high), apiculate (apiculi 2-4.5 microns long, acute), 2-celled; ASCUS 100-140 x 6-7 microns, cylindric to clavate, "apex thickened and with a pore"; PERITHECIUM 250-575 x 200-375 microns (mostly 350 x 300 microns), spherical to ovate or obpyriform, gregarious, "pale yellow to golden yellow, orange or red, turning purplish in 3% KOH", "papillate, papilla typically not well differentiated, when distinctive then 150-200 microns wide, 75-100 microns high"; SUBICULAR HYPHAE 4-6 microns wide, "much branched and often at right angles, septate, smooth", thin-walled, cells often becoming much enlarged and thick-walled, "up to 10-12 microns wide with wall 1-1.5 microns thick", loosely interwoven; CONIDIA (8.5)10-16(21) x 5-7.5(10) microns, at first round or nearly round, becoming elliptic or oboval, (0)1-septate, "with a basal hilum and often with refractive material at the apex", "produced singly and held end to end in dry chains", Blastotrichum-like chlamydospores 18-48 x 12-18 microns, oblong-oval, "1-4 celled, constricted at septa", pale brown to reddish brown, thick-walled, wall 1-3 microns thick; conidiogenous cells "30-65 microns long, 1.5-3 microns at base tapering slightly to the tip", conidiogenous apex apical, "proliferating retrogressively and percurrently, periclinal thickening not observed"; CONIDIOPHORES "arising in the aerial mycelium, indefinite in length", main axis about 4 microns wide, "verticillately branched with three or more branches arising at one point and occasionally repeatedly verticillate", "branches sparingly septate or aseptate, each terminating in a single conidiogenous cell", (Rogerson), SPORES 20-25 x 4-6 microns, fusiform, verrucose (verrucae arranged uniformly, 0.5 micron high), ends apiculate (apiculi 2-4.5 microns long, with acute tips), 1-septate (septum median); ASCUS 100-160 x 6-7 microns, "with apex thickened, penetrated by a pore"; PERITHECIUM (250)350-570 x 200-375 microns, orange, "KOH reaction with the whole perithecium turning purple", perithecial papillae 75-100 microns high, 150-200 microns wide at base (tip obtuse), "of pseudoparenchymatous texture, with cells in divergent files, becoming narrower toward the ostiolar canal and more rounded towards the outer surface"; SUBICULAR HYPHAE 4-6 microns wide (becoming swollen), loosely interwoven; CONIDIA (10)12-17(25) x (5)6-8(10) microns, elliptic to cylindric or oboval, "equilateral, with a central basal hilum", colorless, 1-septate, very rarely also 2-3-septate; conidiogenous locus "proliferating retrogressively, percurrently, forming up to 50 conidia that are held in end-to-end chains"; conidiogenous cells by 2-5(7) in a verticil, 30-65(80) microns long, 3-5 microns wide at base, attenuating to 2-3 microns at tip, "proliferating retrogressively, forming 1 conidiogenous locus"; CONIDIOPHORES "arising from aerial hyphae, hardly or not differentiated from these", 4-7 5m wide, branching irregular to verticillate; aerial mycelium scanty; thick-walled cells "formed in terminal position on lateral branches or intercalarly on aerial or submerged hyphae", yellow or pale brown, with cells 12-20 microns in diameter, subspherical to spherical, held by 3-4, "wall 1-3 microns thick, smooth", (Poldmaa)

Notes:
Hypomyces aurantius is common in north and south temperate regions of the world (Poldmaa). There are collections at the University of British Columbia from BC, OR, IA, and New Zealand. There are collections at the University of Washington from WA, CA, and ON. It was examined from Switzerland, Ukraine, and New Zealand, (Rogerson). It has been recorded from North America (including CA), Cuba, Europe (including Estonia, Switzerland, Ukraine, and the United Kingdom), China, Japan, Kazakhstan, the Russian Far East, Uzbekistan, Eastern Asia, and Australasia (including New Zealand), (Poldmaa).

Synonyms and Alternate Names:
Hypomyces cesatii (Mont.) Tul. & C. Tul.
Hypomyces subaurantius Heinr.-Norm.
Nectria cesatii Mont.