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 and the hyphae are tightly packed in black piedra cases. 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 force applied between or beneath the hair cuticle arises from the growth of the fungus itself. The breakdown of keratin is mainly due to enzymatic processes and corresponds to the abundance of localized mitochondria. The breakdown of keratin begins with the cementing material and progresses to the cortex of the hair shaft. In the cortex two types of degradative patterns are produced which are either parallel or vertical to the axis hair shaft. The parallel pattern arises from hyphal separation of the external cortical layers. The vertical pattern is produced by direct hyphal penetration which creates channels that increase in size as the cortex degrades. Treatment
The infection cannot easily be removed mechanically, although further proliferation of infection can be achieved by avoidance of moisture. Removal generally involves cutting or shaving of the hair, but chemical treatments may be similarly useful. For women some individuals use a fine comb to remove as much of the infection as possible and then they cut or shave their hair. This is then followed by the application of a sublimate solution in 60% alcohol solution to the scalp.
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.