Several mature large boxwoods have patches of brown leaves. I am unable to...
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. Boxwoods have growth buds all along the stem. Hopefully, the plant should put out new growth next season. Monitor the plant throughout the growing season for additional symptoms. See our boxwood publication for more information. http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/HG52_IPM_Boxwood.pdf