Thuja Plicata brown/dead patches

I've grown a thuja hedge for the last 7 years and until recently it's been growing strong and healthy. Then about 6 months ago I noticed some of the scales were turning black on one small part of my hedge in the middle right near the lower part.

Over the past few months this seems to have spread and got a lot worse and now a patch seems to be dead with the scales now turning brown and dying off completely.

Took the liberty of attaching a couple of photos in case anyone has any ideas of what it could be it what's causing it. The only thing that I've noticed is that in the area that's dying there appears to be a lot of spiders webs all over it. I don't know of that's a clue to something that's causing the issue.