If my dentist stopped doing the necessary treatments for periodontal disease a year ago without my knowledge or consent, do I have any recourse?

It depends on several factors, including whether (1) your dentist had undertaken to treat your periodental disease, and (2) if so, whether your dentist had a reasonable basis for stopping treatment. For example, was there an existing relationship under which your dentist agreed to treat your disease? If so, did anything change in the relationship (e.g. your failure to carry out your duties to the dentist and/or conform reasonably to his/her instructions)?