You've got soot mold on your rhododendrons, copy and paste the link below for advice on how to get rid of

http://www.gardeningknowhow.com/ornamental/shrubs/rhododendron/rhododendron-sooty-mold.htm

Check the undersides of the leaves, particularly close to the veins - you've probably got an infestation of Cushion Scale on the plant, and that causes sooty mould to develop on the top of the leaves. At this time of year, you're looking for yellowish brown oval scale insects only 3mm long - there may or may not be whitish, rectangular deposits present, though more usually, these are not seen till later in spring, when its warmer. Best time to treat is late June, early July, when the nymphs (babies) have hatched. Spray with Provado Ultimate Bug killer, or Bug Clear Ultra, making sure you get it under the leaves as well as on top, and spray to run off. The sooty mould should flake off on its own, but you can certainly wash it off with a mild soap and water solution.

There was a tiny white deposit last year but not this. I scraped the white off underneath.Last year I scraped a lot of the black off, so far this year its well stuck. I sprayed with a mixture of washing up liquid & white vinegar & water. A fence has gone up cutting down air circulation.

The plant further away is full of flower buds....I don't know when best to attack !!

Thanks for the replies.

It may be lack of moisture at the roots. These are woodland plants, where fallen leaves keep the surface soil moist. Remember the wonderful displays of them in the big gardens ?

If you saved some leaves to rot down as a mulch
last autumn, (they take 2 years, and should not be mixed with other compost,) - apply now 2" thick. Beech is said to be the most nutritious.
This also applies to Camellias.

Don't use washing up liquid - it is a detergent. You need soap. One recipe is a cup of vegetable oil + 1 tablespoon of soap flakes - mixed to a smooth paste - in a gallon of water.
If you want to be posh you could use neem oil - but it's pretty expensive compared to supermarket own brand cooking oil (just make sure it's 100% vegetable oil)
I buy 100% liquid Castille soap on Amazon but I think you can get 'Liquid Soapflakes' (for clothes washing) in some supermarkets (it's listed at Waitrose online) and Wilkos will sometimes have proper soapflakes.

Soap treatments will not treat for scale, which I strongly suspect is present, though they can be used for the sooty mould. Can't see the point myself, the sooty mould occurs in association with scale, so if the scale's still there, the mould will recur.

The badly affected rd is next to a giant birch tree and a small spindle tree.....a few yrs ago the spindle would get black fly....things changed when next door removed a hawthorn hedge & put up a 5 ft fence....I thought it was an air circulation problem....the soot on some under leaves is really thick ....some black does scrape off

OK. I had a look under the leaves. On the really bad one not seen anything really. On the new sooty one there is just 2-3 of the tiniest scally things...pale brown hidden on the veins...some leaves are sticky a bit...the leaves at the base are black & the soot comes off with finger nail.....some leaves are rusty a bit in need of vitamins I guess.
Before flowering I might scrape off as much scale as I can see. Will sit on a kneeler.....