I have a 20 year old Contorta (Harry Lauder Walking Stick) Shrub that appears to be dying very slowly. Many of the branches are drying up and breaking off; leaves are not coming out as full as it used to, there might be a slight yellow powdery substance on the underside of the leaves and the main trunk seems to have rot at the top of the trunk where it branches out.

I just gave it a thorough pruning but any additional advice would be most appreciated.