Lancashire,
United Kingdom 
rhododendron my plants have black sooty surface and small white chalk like grubs on the underside of the leaf but when you rub them they are just like chalk
29 Jun, 2010
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« why are the leaves on my Caraganas Arborescens turning yellow
hmmmm...two separate but related issues here. The black sooty surface is sooty mould, which will be growing on honeydew that is in turn excreted by sap-sucking insects.
The white grubs are the sap-sucking insects in question, possibly scale insects (or maybe the closely related Mealy Bugs) and their fluffy egg masses from your description - Google "Scale Insects" and see if that looks like what you have.
They are highly resistant to insecticides, but are also immobile, so can easily be removed with a sponge & some soapy water (use vegetable based soap like Ecover to avoid possible damage to the leaf & soil). Be careful to collect them (in a bowl etc.) as you go.
29 Jun, 2010