They've just had the answer on gardening question time, it's aphids that
leave the honeydew residue which attracts sooty mould. Check under your
leaves


John Martin wrote:
: We have a (previously neglected) Camellia in the garden, which a
: black/grey soot-like residue on some of the leaves and a few buds. Is
: it likely to be a fungus? Treatment?
:
: It's possible that the plant could have been splashed while cleaning
: out gutters but the 'soot' doesn't wash off easily
:
: thanks in advance
:
: John