 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.
