Experts and BHG readers answer.
Black Areas on Arborvitae
Discoloration that starts and the base of the tree and moves upward is usually due to a fungal disease rather than winter injury. A couple possibilities on arborvitae are phomopsis twig blight and phytophthora root rot. Is the area where they are planted poorly drained or wet? If so, I'd suspect the root rot. If not, the twig blight may be more likely. Other environmental factors could be causing browning too. It may help to take a sample of the browning (not completely dead) material to your local cooperative extension office for diagnosis. Once the specific problem is identified, you can take the appropriate control measures.