Black dust/sut like deposits

My mother has just noticed that some of her best and biggest plants (including per prized rhodos) are smothered in a black coal dust like deposit. At first, from a distance she thought it was blackfly, until she got closer. Whereas a mildew like deposit turns to powder when you touch it, this is more like a dense black sut.

I have searched and cannot find anything relating to this type of disease; can anyone help, PLEASE.

 

At a guess this sounds like sooty mold - as I recall it is usually associated with the honeydew secreted by aphids. If it is then you need to find the aphids. :(

 

Could it be cushion scale? I found this link:
http://www.telegraph.co.uk/news/main.jhtml?xml=/news/2006/04/17/npest117.xml&sSheet=/news/2006/04/17/ixhome.html
As you don't often see aphids on Rhododendrons, thought this might be the answer. Get your Mum to look for the scales on the undersides of the leaves. It is hard to treat, the scales aren't easy to kill and you have to wipe them off, then treat the young ones (nymphs?) later.

 

I've just looked up sooty mould in the RHS pests and disease book, as I agree with Fran that this is what it sounds like, but, like Liz, I didn't think Rhodies would be affected. It says that you often see it on plants growing beneath ones which are affected by the aphids, etc - and it's exretions from there, falling down. So check what's above the rhodies, Tracey-Anne!

 

Thanks to you all for the information. Now I have a name, I have looked again and have found a good link regarding sooty mould. It is:

http://plantclinic.cornell.edu/FactSheets/sooty/sootymold.htm