Greetings Christian:
Unfortunately, like 90%+ off all the tree health problems I receive via this Internet venue -- my answer is that you are going to need to contact a local professional forester, horticulturist, or arborist for an on-site assessment.
Without physically seeing the tree and knowing more about the general environmental conditions, it is extremely difficult, if not impossible, for me to provide you with a legitimate and valid recommendation.
Therefore, please contact a local professional forester, horticulturist, or arborist for an on-site assessment of your tree(s).
Best of luck!