Posted: 18/04/2012 at 20:58
I wonder if this is sooty mould, although I've never seen it on my rhodo, azalea or pieris before, but have had it on my camellia.
Sooty mould develops on the surface of leaves when sap, or honeydew, drips down onto the surface from sap-sucking insects above. Scale insect is the usual culprit.
Sheddy, take a look at the underside of the leaves above the sooty ones and see if you can find scale insects feeding. If you have then you'll need to try and get rid of these pests in some way (not easy), but if not we'll have to think again. Let us know what you find.