In article , solaara solaara.1qxy80@
gardenbanter.co.uk writes

my next door neighbour has a bay tree with sooty black stuff on the
leaves..whats the cure?

Check for scale insects - the sooty black stuff is mould probably
living on excretions resulting from infestation - and deal with these.
Depending on size of bay tree, either pick off the scale insects by hand
or use a systemic insecticide - but don't use the bay leaves in your
rice pudding any more if you use the insecticide.
--
Kay
"Do not insult the crocodile until you have crossed the river"