[hampan]

 A clump of old (30 yr) bay leaf trees have for several years had leaf spot, first on the backs of leaves and now on all. They're probably 10m tall so no idea how to treat. This year lots of dry leaves on the ground for the first time. The trees are still full of leaves but whatever the problem is seems to be getting worse.

[David Dickinson]

Sorry hampan I can't really help you. Just replying so that you know we are seeing your message. The tree I have behind my garden is fine but trees a few metres away which are in almost full shade never look healthy. Could light intensity have something to do with it?

[John J]

Hi hampan, I'm no expert on this subject but I believe there could be a number of reasons for your problem.
Several insects, aphids, etc, can excrete honeydew onto the leaves which can cause spots to appear.
Bacterial and/or fungal diseases could cause leaf spots.
Even sunburn, believe it or not, but that's highly unlikely.
Can I suggest you Google something like www.gardeningknowhow.co/edible/herbs/bay/sweet-bay-leaf-spots.htm.
Good luck finding an answer.

[Fermi]

Looks a bit like spider-mite damage
cheers
fermi