I have a 2 thuja occidentalis smarged, 11 years planted in my back garden they are about 13 feet tall, and planted quite close to each other, they’er in a sheltered spot near a corner wall, there is euphorbia planted in front of them, they have turned brown in alot of places and when I touch it lumps of it fall off, this brown is spreading to the top. about 30 percent of each tree is affected. Local gardener told me to treat them with epsom salts disolved in 2 gallons of water which I did today.
Close conditions can also cause dieback due to increased dampness and more fungal activity.
Increasing air-flow and light would help and this might mean removing some plants or pruning them up to allow more light in.
Fungicides are not a solution as this is due to the conditions in which the trees are growing.
A attack of greenflies is another possibility and can cause dieback, so keep a check for these.