East Sussex, United Kingdom

I have recently discovered that my rhododendrons have succumbed to 'sooty mould' and large areas are completely covered in the black mold. The rhododendrons are over 4 meters high, 3 meters deep and over 60 meters in length so I have lots of work ahead of me!
I have so far tried spraying them with 'Ecover' detergent to try and loosen the black deposits after looking on google but this did not help. I have been left with no option than using a bucket of soapy water and a sponge and working my way along the plants which is a soul destroying job. As you'll see from the attached photo, the picture shows the leaves before and after cleaning.

Can anybody tell me if there is any other way of removing this mould as it is literally 'baked' on to the leaves. Once cleaned, the beautiful leaf is still there though I am worried that these will soon start to die from lack of sunlight.

Any suggestions will be gratefully received!

Update :

Firstly, a big thank you to all of you for your help in this. It's been a bit overwhelming due to the amount of rhododendrons and the extent of the problems I am finding.
I have now checked the leaves and have found that many of the undersides of the leaves are covered in scale insects as you will see from the photograph. I've also seen quite a few minute insects crawling about too that I imagine are newly hatched.
I can see that I will need to urgently spray the rhododendrons but how will this be effective if all of the infestation is present on the underside of the leaf? Does the pesticide need to coat all leaf surfaces and if so, how do I achieve this on such huge plants?

On plant Rhododendron

• 3 Apr, 2012