By Dunno
United Kingdom 
black soot like substance on pieris formosa
10 Apr, 2014
By Dunno
United Kingdom
black soot like substance on pieris formosa
10 Apr, 2014
Check the woody stems for signs of scale infestation - it sounds like you have sooty mould, and that's often/usually associated with either aphid infestation or scale insect. Check backs of leaves and stems too.
10 Apr, 2014