Help! Pieris (Forest Flame) leaves have all gone black and sooty!

[scarymamma]

I've had a Pieris in the garden for about 5 years and it's growing really s-l-o-w-l-y. I did some weeding today and noticed that all the older leaves are covered in a black sooty type stuff. We're not near a main road and I suspect it's some sort of fungal/mildew type infection. Anyone heard of something similar. know what it is and know how to treat it? I've done a quick Google search and can't find anything similar.

What are the undersides of the leaf like? Can you see little bugs there? Sometimes, an infestation of bugs can encourage the growth of sooty mould, which grows on their (excuse me) poo. Look at the RHS advice on cushion scale.

If you don't think it's that, take a few leaves into a garden centre and ask for their advice.