you need to wash the leaves with a soft soap [not detergent] and then treat the cause. It is often a secondary fungal 'infection' feeding on the sugary honeydew from pests like aphips etc. if you use soft soap that will also help kill the pests too.

hope this helps.

Too the words right out from under my fingers Sbg :-)

Is the plant in question a Bay tree by any chance?

guest wont be able to reply to us Fractal. its one of the conditions of a guest question.

I see, thanks Sbg.