Affected hairs develop stone-like black nodules affixed to the hair shaft that cause weakness of the hair. Infected hairs treated with potassium hydroxide fluoresce under ultraviolet light despite that the fungus itself does not normally fluoresce. Fluorescence of the piedra indicates secondary contamination by bacteria. 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. Pathology
Piedraia hortae causes the formation of nodules on the hair shaft, a clinical superficial disease commonly known as black piedra. Black piedra is usually seen in tropical regions and it usually targets humans  of all ages and targets the scalp, moustache and occasionally pubic hair. The source of the infection is usually in soils, poor hygiene, long hair, cultural use of veils and the application of plant oils to wet hair favours the growth of the infection. Black piedra is a superficial fungal infection, which means that it is restricted to the stratum corneum and causes no inflammation. The infection of the hair shaft results in the formation of nodules on the scalp, moustache and pubic hair. The nodules are hard and gritty, which produce a metallic sound when the hair is combed. The nodules colonize the hair shaft, which causes progressive weakness of the hair and leads to breakage of the hair in severe cases, which can lead to hair loss and baldness. The fungus also has the potential to destroy the cuticular layers of the hair and move into the cortex.
Historical treatments have used alcoholic tinctures of heavy metals, such as mercury bichloride. The application of antifungal shampoos such as pyrithione zinc, formaldehyde and salicylic acid is effective against black piedra. Oral therapy with itraconazole or terbinafine also causes nodules to break down over time. Removal of affected hair and treatment with topical agents is also effective and results in very low recurrences rates. However, even in the absence of treatment, spontaneous remission may occur.Terbinafine has been used in the treatment. Cosmetic uses
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. Several Indian tribes located from Panama have been known to use several methods in order to darken the hair of albino individuals within their community. One of these methods is the cultivation of black piedra for an extensive period of time in the individuals hair. In Malaysia the nodules of black piedra are very attractive and women are encouraged to sleep with their hair buried in the soil to encourage growth of the black nodules. Similar taxa
The genus Piedraia contains another species known as Piedraia quintanilhae, which is more common in chimpanzees than humans. It differs from P. hortae in terms of the ascospores do not have any attachments. Another species known as Trichosporon biegelii is commonly known to cause white piedra . White piedra is more common in temperate and semitropical climates, such as South America, Asia, Europe, Japan, and parts of the southern United States.