Longitudinal Melanonychia is a serious condition. I would strongly suggest that you see a dermatologist
soon in order to further evaluate this situation.
Longitudinal melanonychia, also know as Melanonychia striata, is characterized by a pigmented stripe of tan or brown or black area within the length of the nail bed. This can occur from a variety of causes. Sometimes simple trauma to the nail looks similar. The disease itself can be caused malnutrition, many different medicines, vitamin deficiencies, or cancers. Occasionally endocrine tumors can cause this. Rarely, infections such as fungal infections can cause a similar appearance. However the most significant concern is if this longitudinal melanonychia represents a skin cancer underneath the nail bed. This is known as subungual melanoma.
To answer your question, this should be treatable. However, it very much depends on what the cause of the longitudinal melanonychia is caused. As mentioned above, some of these disorders are very treatable. Some are very serious, and some are minor and require no intervention.
Diagnosis is key. I would suggest you see a dermatologist (skin specialist) soon. He or she can look at the lesion as well as see if you have any other symptoms that may be related. Occasionally a biopsy
of the skin is needed for further investigation.