Well, it took a while but now we are sure we have identified the problem with the topiary buxus in our garden.
We knew it was Box Blight and now we also know the fungus is Cylindrocladium Buxicola. It is rather new, seen first in The Netherlands around 2005 and in our garden in 2009. The photo below shows the very beginning...
Taking into account the number and size of topiary box it can easily ruin our complete garden.
I now started a treatment with a new fungicide so it is now, apply, wait and see.
If you want more details have a look to my recent blog-post Buxus disease, Box Blight? Cylindrocladium Buxicola, continued.