It isn't likely that you are having a problem with Phytophthora unless your planting was new.
There are multiple things that could cause dieback on Rhododendron branches, including borers, Botryosphaeria canker, rodent or vole damage, etc.
Here is a page that lists the most common problems. Take a look and compare:
How much of the shrub is affected? We would expect some dead branches in a shrub of that age.
If you have further questions after reading the linked publication or you have photos you would like to share, they can be attached directly to this reply by using the tabs below.
The scattered dying branches looks like it could be Botryospaeria disease, but it certainly could include other things like borers as well.
Read through the publication shared above which tells you what to look for and what to do.