By Sjw
gloucestershire,
United Kingdom 
I have several clipped box in large containers, some of them seem to have what appears to be a sooty substance on the inner branches, they are not dying but the leaves do not look as healthy as they have done, they were repotte about a year ago and are fed growmore and have had new soil added to the top. Have sprayed with a anti fungal but does not seem to have worked.
5 Jan, 2011
Answers
Suggest the sootiness might be sooty mould, formed on honeydew from aphids feeding on the youngest growth. It's probably old mould,formed when weather was warmer. Not common with box, as they're normally pretty much able to shake off anything. Have a check on the undersides of the growths to see if there are scale insect on the leaf midribs and stems. if there is something there, it'll be an insecticide that's wanted, in a couple of months time. Worthy
5 Jan, 2011
Anti fungals are much more effective if used in Spring and summer, the air temperature makes a difference to their effectiveness.
Nothing springs to mind as an explanation - post a photo of the problem if you can, or just wait and see what happens by Spring. I'd ask if the plants were covered in snow for any length of time? If so, that might cause this, and it'll pass if it is that.
5 Jan, 2011