Where in the world are you?

kimmq is kimmsr

Danny Clark, I do not know the plant well enough to be specific; but the evidence (about 400 plants) suggests an intensive operation. It is easy for a fungus (my guess) to proliferate under such conditions. That you have had to cull 50 plants indicates an infection that has taken hold. My suggestion is to positively identify the causative organism; separate the remaining plants into 3 well defined areas. Space them adequately to facilitate air circulation. Classify them as "Good" "Going" and "Gone".

Obtain a chemical (I cannot think of an alternative) which targets the pathogen/pest and,as a bonus, also lists the site (Boxwood) on the label. Apply to all three groups at the curative rate. Thereafter, apply at varying intervals to each group. The worst affected group can be treated at the shortest interval; and the "Good" group at the longest interval. Observe at weekly (at the very least) intervals; shift plants from one group to another as you see appropriate; and cull - ruthlessly!

I wish you luck - sincerely! Because I know from experience, that you will need some of that also.