Zigga, spraying will do no good. ,
the cause is aphids. Their secretions help sooty mould to form
bit of a job but you meed to physically wash as many leaves as you can THEN you can spray with an insecticide
i had a choisya, skimmia and sarcoccoca covered with mould ....maybe 4 years ago. Every leaf, just about, was washed top and bottom. i lightly pruned back all bushes to start with then used an insecticide. a soap spray a couple or more times during the summer comtrolled further aphid attacks.
my bushes have been clear ever since
cheshire gal, a mildew killer will have no affect but try pruning back to a more manageable size to again attempt to wash your rhodo.