We've all heard about Box Blight, which is ravaging across the land, ruining decorative clipped Box hedges left, right and centre, but it's worth mentioning that there are more diseases than just Blight, and not all of them are as bad.
Last year I was asked to look at a sickly hedge, whose owner had assumed was being ruined by Blight - she was panicking about having to rip out the wavy hedge that has taken years to clip to shape, just exactly how she wanted it.
Rather than reaching for the axe, I persuaded her to take a very close look at the damage. As you can see, the hedge has a lot of light brown foliage, and a closer look shows them to have round straw-coloured areas, which appear to be gradually spreading across the leaves. The underside of the leaves had lots of raised black pustules (“lovely!”) as well.
This is not Blight - this is Box Rust.
Superficially similar to Box Blight, but very treatable: we sprayed the hedge with Bayer Fungus Fighter (there are other products out there, I'm not being paid by Bayer, honest!) and left the hedge for a month or so: I then clipped it as normal, taking great care to shake all the loose clippings - which were, of course, mostly the affected foliage - out of the plant, and to clean up the debris really well afterwards. The owner had one of those garden vacuum things, so we used that to do a really good job.
The hedge recovered and now looks superb - so if you find areas of brown foliage on your Box, don't panic too quickly: get a hand-lens or a magnifying glass and take a close look at the leaves, particularly on the undersides.