my rhododendron's leafs are turning brown. what have i done wrong and how can i remedy the situation?

If you live in zone 4-5, its now a normal thing, as fall is approaching and they are going dormat.

Its called Phytophthora dieback (fungus). Terminal buds and leaves turn brown. Prune and remove affected branches. Avoid overhead watering, watering late in the day or other practices that prolong periods of leaf wetting. To prevent re-infection and protect the remaining plant, drench or apply a foliar spray of Fosetyl-Al or mefenoxam. I had that happen to me also. Good Luck