I have a very long small buxus hedge , for no reason one branch goes yellow wilts turns brown and dies.
If this is left the other branches soon follow i have not seen this before and attacks randomly, most nurseries can not tell me anything.
Have a look at this site;
and try the site below.
Here is a link that might be useful: Major diseases of boxwood (buxus)
thanks Robyn 6014 great info