Horizontal lines on the fingernail caps may be caused by psoriasis, onychomycosis, perinail warts and other diseases. 1. Psoriasis: the cause of psoriasis has not been perfected, and it may be caused by a combination of genetic and environmental factors. The patient’s nails will appear thimble-like, point-like depression, which can be manifested as horizontal lines on the nail cover. 2. Ringworm of the nail: ringworm of the nail is usually caused by dermatophyte infection, or may be triggered by factors such as nail trauma and impaired immune status. The nail plate of the patient may show turbidity, separation, atrophy, buckling and other symptoms, manifested as horizontal lines on the nail cover. 3. Periungual warts: Periungual warts are mainly caused by human papilloma virus infection. When the warts are too big to cause the nail plate to be deformed, some patients will appear to have horizontal lines on the nail cover. There are a lot of reasons for this, so you need to go to the hospital and find out the cause of the problem.