Posted: 04/06/2014 at 18:49
It could be a break in the stem to those branches or a break in the bark layer which is exposing the cambium layer and preventing moisture and nutrients flowing up the stems. It may also be verticulum wilt or simply severe thirst?
Cut off one of the sick stems and check for verticulum wilt - http://www.rhs.org.uk/advice/profile?PID=255 If not that, try giving it a good soaking at the roots to see if that helps it recover. If not, cut out all the affected stems and give it a feed or liquid rose or tomato food and some sequestered iron to help it recover.
If the pruned stems do show signs of verticulum wilt you need to remove the plant with as much root ball as possible and destroy it. Do not plant another rhodo in its place. Disinfect your secateurs and other tools after cutting and after digging out.