Rhododendron looking very poorly
We have a 40 year old plus Rhododendron which this year is looking very sorry for itself. Less leaves have formed and what leaves there are, are drooping and have a coating of a "black soot". The flower buds are poorly formed and dropping before opening. We have given it a feed of a Miracle Grow specialist rhododendron feed and are watering each night . Any ideas would be most welcome as we would hate to lose this matriarch of a tree from the garden.