Should I Disclose Mental Health Conditions to a Potential Advisor Before Applying to the Program?
I fear flying.


Nearly all PhD students go to conferences.  They usually go there by flying.  There is no way to hide that you fear flying.  Therefore, I suggest disclosing so that your advisor can arrange for your to participate in conferences without flying.  Disclose before you enroll, and find out if your advisor can help you be successful before you enroll.

Should you disclose before you apply?  It depends.  If you can disclose in a way that shows you will be successful in the program (e.g. "Following treatment, my symptoms are improving so I expect my future achievements will be better than my past achievements...") that may increase your chances of admission.  Show you have a plan for success.  If you disclose and indicate you cannot be successful in the program, then you probably will not be admitted.  But why would you apply if you cannot be successful?

Anecdote:  One of my colleagues disclosed a fear of flying to me when he was a PhD student.  He is now an assistant professor.  He does fly from time to time.