sooty deposit on ericaceous plants

I have a black sooty deposit on 2 pieris, and several quite large rhododendrons. At the nursery they said I could just wash it off - this is all very well, but they are quite big plants and it would take me for ever! Is there anything I can spray which will do the trick?

Sooty deposit

Reply from Tracey James

I think the sooty deposit is the waste from scale mites. You may find small white dots on the opposite side of the leaves. A regular pest control product that clears scale bugs should work and the soot or rust will wipe off (however much you can manage - it's worth it) or eventually clear with rain and general weathering once the scales bugs have been dealt with! I have had the same problem with several camelias. Looks terrible but is quite easy to deal with.
Good Luck

from 'Mary,Mary' in Somerset