What causes branches of my bushes to die off? Insects? (haven't seen any evidence of any sort of critter) Disease? It isn't noticable till the leaves on that branch start to turn brown - by then the branch is dry and brittle and snaps right off. Is there a systemic potion of some sort that I can add to the soil to combat this problem? I've got both American and English boxwoods, but the English seem to be more prone to the problem. FYI, the bushes are probably 30 years old (minimum).

Perhaps this site will help,

http://www.ext.vt.edu/pubs/plantdiseasefs/450-614/450-614.html

The drought in out part of NC is further killing off parts of the bushes now. I guess the plants shut down branches in order that the plant may survive. I guess we need a week or so of steady drizzle to get things anywhere near where they should be at this time of year.

I had similar problems with my boxwoods. I did some research & although I never came to a conclusion as to exactly what my problem was, it became clear that this shrub is very susceptible to a variety of ailments & is not a very good shrub. I replaced all my boxwoods with dwarf yaupon hollies & have been very pleased...they look very similar and are virtually problem free. I don't know whether or not they are suitable for your part of the country, but it would be worth looking in to.