Kay Easton wrote:

In article , Camilla
Peake writes
Can anyone inform me what the black sticky substance might be that keeps
appearing on my bay trees. They are outside in Summer and in an unheated
conservatory in the Winter. It drops down onto the floor tiles around the
pots.
Have tried washing with mild soapy water but, alas, it keeps returning.

It's mould growing on the excretions from sap sucking pests.

Possible candidates are aphids, though the soapy water should have seen
to them, or scale insect. You would spot the scale insects as small oval
brown scales on the undersides of the leaves along the veins. Difficult
to get rid of because of their tough coats - I remove them by hand but
on a bay tree of any size that is impractical.

And be careful using smothering agents like Safers soap or detergent spray on
Bay trees. I managed to defoliate mine entirely this way when treating a bad
infestation of scale. The leaves lost their waxy coat and desiccated so badly
it died back to its roots. Amazingly it regrew and the scale insects did not
recurr.

Otherwise use a systemic insecticide.

But not if you intend to use leaves from it for culinary purposes.

Regards,
Martin Brown