In human medicine, onychomycosis is a fungal disease of the toe- and fingernails. In horses, various species of fungi are often found in the crumbly, disintegrating horn typical of white line disease and seedy toe (and the hoof wall separation which results). It is as yet not certain whether these fungi are the primary cause of the disease, or are opportunistic pathogens than ‘join in’ after other factors or pathogens have caused the initial damage.

Onychomycosis is sometimes used as a synonym for one or both of the above mentioned disorders, but this is rather misleading, as it is unlikely that a fungal origin alone is responsible for the damage to the inner layers of the hoof wall.