Nothing can reverse the browning and dieback, so affected branches need to be pruned off. We can't tell what caused it since several factors can cause the same symptoms, including drought (we're still quite dry in most areas of the state, and have been for almost two years), over-watering (if the planting bed is irrigated regularly), vole gnawing damage (especially in winter and/or under snow or mulch cover), exposure to too much ice-melting salts or branch breakage from snow/ice load (especially if anything was shoveled onto them), and opportunistic fungal infections that take advantage of stressed plants or those whose foliage is often wet from rain, irrigation, or dew. Winter damage can take a few weeks to show up on evergreen foliage in some cases.

When you trim off the brown branches at their base, you can look for indications of darkened or split bark (which might suggest a fungal canker disease) or missing bark (which would point to voles) to narrow-down the cause of dieback. If the planting bed has not been irrigated in a long time, you may want to water it thoroughly to rehydrate the plants to avoid future damage resulting from drought stress.

Miri