Rhododendrum with black leaves

My rhododendrum has been beautiful in the past yet noticed that it has black leaves at the bottom in particular. It is well established, 6ft in height and it seems to look like soil splattered on the leaves but is not. Should I prune it to get rid of this black as at the top there does not seem to be any. Was definitely not looking this way this time last year!!

 

Hi it could be a couple of things old leaves , leaf spot fugal infection which I think it is, what I would do is cut them all off , put in the bin not the compost heap , clear away all the old leaves from under the bush and give it a good spring clean re-top dress with a good mulch , give it a feed of Rhodi fertliser, you can also spay if it is black spot , I think it is manconzeb is the one to look for , do late evening so the insects dont get a spray.
Hope this helps

Spruce