A new species of Phaeocalicium (Mycocaliciaceae) on black walnut in central North America

Phaeocalicium atenitikon is described new to science, growing on twigs of Juglans nigra (black walnut) in central North America. The species is characterized by small ascomata; an exciple of brown, periclinally arranged hyphae; notably small, pigmented, two-celled ascospores with pale septa; and its substrate and distribution. These are the first records of a corticolous Phaeocalicium from intermountain North America south of the Great Lakes region. Differences among similar species are discussed, and a worldwide key is provided for members of the genus with persistently 1-septate ascospores.