Your description does not sound like a problem caused by disease. In particular, it does not sound like the very infectious boxwood blight which is active in warmer temperatures and would typically result in yellow leaves on the ground rather than remaining on the shrub. Timing (March) suggests that this is a common winter injury problem. There is something about the site that is less hospitable to the plants than to your others. That could be more wind exposure, proximity to paths or street that could result in de-icing salt being splashed into the soil (boxwood have very shallow roots and are really vulnerable to salt burn), more sun exposure, poorly draining soil, compaction of soil around the roots or mulch that has blown away and exposed delicate roots. All those problems lead to drying and dieback.

Boxwood does not reliably regrow from bare wood, so cutting the plant back to healthy wood is not a strategy you can use to improve their condition. It also seems likely that this is just not a good site for these plants or site improvements should be made before replanting boxwood here. I think that, sadly, disposing of the plant is the reasonable option.

There are some indicators you can look for that will tell you whether there is possibly some disease present on these plants that could be communicated to other boxwood, like the presence of black spores on the twigs or lesions on the stem near the ground. If you see either of those things, they will probably be opportunistic diseases that have taken advantage of a weakened plant rather than the source of its problems. You should bag the plant materials just in case and dispose of them outside of the garden.

Use this link to our Guide to Boxwood which will tell you more about common pests and diseases of boxwood in the northeast, help with siting and preparing the garden for new boxwood as well as some less problematic types to consider in case you want to start again.

Courtesy of NYBG Plant Information Service