Something is killing parts of my blue rug junipers. Many of the twigs on the branches are going brown. I've cut back repeatedly and used Bordeaux Mixture to kill any possible disease, but it keeps happening.

The plants are several years old and grow at the top of a retaining wall, draping themselves several feet downward over the wall. They get full day sun.

What could be doing this? I live on Cape Cod, if that helps.