Answer: Based on your description I suspect it is winter injury. This can be caused by many factors including temperature fluctuations, especially when plunging cold temperatures follow a mild spell, or by exposure to unusually cold temperatures, or by excessive wind. Late season fertilization can also exacerbate winter damage by encouraging late season growth that does not have time to harden off in preparation for the coming cold winter.
Since pruning can alter the overall shape of the plant for years to come, I would urge caution at this point. If the wood is truly dead it should be removed, this would be branches that have no green inside, are brittle and dry; dead wood will not regenerate so it should be cut off neatly, cut just above a side branch to avoid leaving a stub. There is also the possibility that some of it may be discolored as a natural response to cold and could recover, so be patient before removing marginal areas.
This spring, fertilize with a general purpose granular fertilizer per the label directions and then water this summer during dry spells to encourage regrowth.
Since I have not seen the plants, it is a bit difficult to make a diagnosis long distance. For this reason I would suggest that you also consult with your county extension and/or with a local, professionally trained and certified nurseryman. Based on photos and an awareness of the local weather patterns and the planting site, they would be better able to diagnose the problem and based on knowing that, suggest how to proceed.
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