I bought a bay tree around two years ago and it's never really thrived. I have repotted it into a large pot, making sure it had good drainage, filled it with good quality soil and fed it plant food.
It has improved a little, but not much.
The leaves seem to be dirty with a fine black soot on them, as opposed to bright, clean and green. Is this disease? How can I help it thrive?
In my experience the only thing that attacks the bay tree is the scale insect usually when kept in a poorly ventilated area so it may just need a "scrub up".
What I have also found is that they take a bit of time to recover from any bad experience so have patience.