Rhododendron Mould?

Hi, I noticed while gardening today that all my rhododendron bushes are covered in black stuff on the leaves.  It looks like it would wash off, or maybe mould or polution (!) even though we don't live in a heavily poluted area!!!  I have a feeling that it is some sort of disease, but have no idea what and don't know what to do about it.  Virtually all of the entire bush is coveredand I have several with the same problem and as they are mature bushes, that is a lot of infected leaves!  Before I start chopping them down, which will leave massive gaping holes in my garden, can anybody tell me what it may be and how to tackle it.  Your help would be much appreciated!  Thank you x

The plant has probably got scale insects look carefully on the under sides of the leaves above the black mould, the scales are usually seen along the center of the leaves and down the stems if you find them you will have use a spray to get rid of them.

Thanks for your reply - I hope this spray is cheap as I have several of these bushes, all are infected and most are about 9' high and are infected all over!  I had a look and they do have white scaley patches on them, so I'm guessing you're right with your diagnosis.  I'm wondering if chopping them right back might be an option and spraying what's left!  They are a bit overgrown at the moment after all - more suited to a stately home than  my semi-detatched back garden!

Rhododendrons don't like being cut back hard, in spring it's ok to trim old and straggly stems. Also the black mould should gradually get washed off by the rain after you have treated the scale problem.

Thank you for your advice - wish me luck getting rid of the scale - I think I may need it!!