The most likely factor by far this past season is drought; much of the state was in drought status for most of the summer, and plants not receiving irrigation suffered greatly. We have seen many instances of dead arborvitae due to the drought in areas where we know the plants were not watered, or not watered enough. Did your client irrigate periodically (about one soaking per week when there wasn't rain, roughly)? If not, that is the most likely reason for plant death. Evergreens do not show obvious signs of drought stress like wilting or discoloration until it's too late to remedy. The soil needs to be checked about six inches deep, and if it feels somewhat dry to the touch at that depth, watering is needed. (If damp when checked, watering can wait.) It is possible to over-water during a drought if someone irrigates too heavily or too often, but under-watering is more likely, especially if irrigation was too light, too infrequent, or lacking altogether.

Some fungal cankers, like Botryosphaeria, can attack shrubs and trees that are stressed, and that stress often centers around drought. Arborvitae are not very drought-resistant plants compared to evergreens like Junipers; they are native to habitats with more even sources of moisture. This would be an above-ground issue if Botryosphaeria is involved; it does not infect roots, though plants with dieback this extensive are not salvageable even if their roots are okay. Opportunistic wood-boring insects, like certain beetles, can also invade stressed Arborvitae and other trees, but they are not the primary issue and cannot be treated once they've tunneled into the wood. Such insect and fungal issues finish-off a stressed tree; they don't initiate the problem, which was environmental stress.

You don't mention what the "tree and shrub service" entails, but unless it was monitoring plants for watering needs, that should not be a factor in their decline. Fertilization and routine pesticide applications are not warranted or needed for the majority of tree and shrubs in home landscapes, Arborvitae included. Lawn fertilization would not be a contributing factor to this dieback unless the roots were exposed to excessive fertilizer, which if that were the case, "burn" would be evident in the lawn itself, which it is not. If the browning didn't manifest until after the lawn fertilization, it was just a coincidence in timing. A delay to browning for evergreens lacking water is one of the main reasons such plants are used for holiday decor; the rootless branches and cut Christmas trees stay green for a reasonable amount of time after they are doomed to dehydration, having no way to absorb water to replace what the needles are losing. The point of no return for these Arborvitae could well have passed weeks or even a couple months ago, and recent rains were not enough to counteract that damage.

Now is a good time to replant if the client would like these replaced (since they can't recover). New plants should be monitored for watering needs for the rest of fall and even into winter (during mild spells) since evergreens do still lose moisture during the dormant season if snow or rain is insufficient.

Miri