Posted: 09/06/2016 at 10:13
To be honest Jim - I'm not sure at all.
I have some brown bits like that where fresh growth was a bit nipped by the frosts. I also know that occasionally a bit of weedkiller spray might reach the hedge when I'm leaning over to try to zap the huge nettles and thistles which have been allowed to grow wild in the field behind my garden.
If you don't think either of those is the problem for you, could the hedge perhaps be very dry and / or hungry?.
It looks as though there is quite a lot of stuff planted up to the base of the hedge. Maybe clearing some of that away, giving it a feed, a good watering and mulching would be beneficial. You could also give it a foliar feed (liquid seaweed) a couple of times this season.