Long horizontal lines on toenails may be caused by toenail trauma, or by diseases such as onychomycosis and psoriasis. 1. Toenail trauma: if the toenail has recently been squeezed and injured, causing damage to the nail plate and resulting in a change of form, then the toenail will appear to have long horizontal lines. 2. Nail fungus: the main causative agent of this disease is dermatophytes, commonly found in red trichophyton, trichophyton fungus, etc. If you suffer from tinea pedis, it is also easy to cause tinea pedis, and common triggering factors are impaired immune function and frequent wearing of airtight shoes. Typical symptoms are nail plate can be manifested as transverse lines, unevenness, turbidity, thickening, separation and so on. 3. Psoriasis: the etiology of this disease is still unclear, studies show that it is considered to be the result of a combination of genetic and environmental factors, such as viral or bacterial infections, obesity, smoking, skin injuries and so on. Common symptoms include the presence of hyperkeratosis and separation of the toenails, which can be manifested as long horizontal stripes on the toenails. When the toenails grow horizontal lines, you should go to the hospital in time to improve the relevant examination to clarify the specific causes.