Piedraia hortae is a superficial fungus that exists in the soils of tropical and subtropical environments and affects both sexes of all ages.

[2] The fungus grows very slowly, forming dark hyphae, which contain chlamydoconidia cells and black colonies when grown on agar.

[citation needed] When grown on agar at 25 °C (77 °F) Piedraia hortae grows very slowly[4] to form black-greenish, limited and pointed colonies.

[8] Identification is easily achieved by microscopic examination of the hair nodules, and can be confirmed by sequence analysis of the nuclear ribosomal internal transcribed spacer region.

[9] Piedraia hortae causes the formation of nodules on the hair shaft, a clinical superficial disease commonly known as black piedra.

Piedraia hortae survives in the scalp is due to the slow rate of the keratin degradation near the cortex and the compact formation of the nodules[6] and the hyphae are tightly packed in black piedra cases.

[12] The initial invasion of human hair by P. hortae is achieved by using an eroding hyphae, which force their way beneath or between the cuticular layer.

The vertical pattern is produced by direct hyphal penetration which creates channels that increase in size as the cortex degrades.

Black piedra is sometimes cultivated for cosmetic purposes due to social factors that favour a specific hair colour, which makes them more attractive in their society.

White piedra is more common in temperate and semitropical climates,[20] such as South America, Asia, Europe, Japan, and parts of the southern United States.

[20] Recent studies have shown that the black, lichen-colonizing fungus, Xanthoriicola physciae, is closely related to P. hortae.