British Columbia: Leggy Rododendrum

We have a huge rodo that has a beautiful canopy of flowers, but the inside is bare and spindly and it's full of black spot it's never being looked after, it's about 10ft. tall and we're thinking about cutting it right down to about 3ft. tall in the fall. We are hoping it will come back with some TLC. Are we on the right track or are we going to kill it.

 

Should be done right away instead of in fall. However, if problem is the now common foliage mildew specimen may get worse, even die in response to mildew + cutting down, instead of improving.