"Wynnea americana" is distinguished from other species in the genus "Wynnea" by the pustules (small bumps) on the outer surface, and microscopically by the large asymmetrical longitudinally ribbed spores with a sharply pointed tip.The spores are made in structures called asci, which have thickened rings at one end that are capped by a hinged structure known as the operculum—a lid that is opened when spores are to be released from the ascus.