Posted: 24/09/2017 at 12:10
My best bet would be a fungal root infection, either honey fungus or phytophthora:
Treatments aren't available so the best bet is to remove all affected parts of the hedge plus the next unaffected one, as that is likely already infected. The affected soil will need to be dug out and replaced unless you plant different species which are immune. Not good news, I know.
Last edited: 24 September 2017 12:10:38