Boxwoods in general can be susceptible to several fungal diseases, insect, and abiotic issues (cultural and environmental). Please send us photos of the affected boxwoods. Send photos of the whole shrub and affected branches and stems so we can see what you are dealing with. Look at our publication for information on symptoms and causes
The boxwood in the photos look like they were susceptible to volutella, a fungal disease. The browning of the branches on the boxwood looks like volutella, a fungal disease.
At this point, all you can do is prune out the affected portions. Remove all old diseased leaves in the shrub and on the ground. Proper pruning includes thinning. This opens up the plant to good air circulation and sunlight. Thick foliage encourages fungal diseases. See our boxwood publication for more information.http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/HG52_IPM_Boxwood.pdf