Boxwood is one of the most popular evergreen woody shrubs in the eastern U.S. It is one of the more frequently submitted plants to the UConn Plant Diagnostic Lab, typically with symptoms of browning and/or dieback. Most clients are worried about the recently introduced boxwood blight, a serious disease caused by the fungus Calonectria pseudonaviculata (synonym Cylindrocladium pseudonaviculatum). In most cases this is not the problem.
If your boxwood have developed some browning, off-color foliage and maybe some defoliation or dieback on severely affected branches, take a close look at the leaves, both brown ones and adjacent green ones. Check for tiny white dots or marks as shown in the photos. These are sites of boxwood mite feeding injury and it was quite common last year (2015).
The boxwood spider mite (Eurytetranychus buxi) is most active during spring and early summer but browning may occur weeks or months later. If injury is observed, check for active mites before treating plants by tapping a few symptomatic branches (during the growing season) over a stiff white paper or card. Paper plates work great. Tiny, slow-moving tan-colored mites can be observed if present.
Naturally occurring predators may help with control but, if needed, many mites can be removed using a strong stream/spray of water. Other remedies include horticultural oil and insecticidal soap. January 2016.