By Jnicholl
United Kingdom 
my rhododendron bush has a black coating on the leaves and has, what looks like, white powdery rice shapes on the underside of the leaves.
27 Aug, 2013
From the description I'd say Cushion Scale and Sooty Mould.
Treatment advice given by RHS can be found here
http://apps.rhs.org.uk/advicesearch/profile.aspx?pid=483
27 Aug, 2013