Answer: It's really difficult to diagnose a plant problem without actually seeing the plant. Arborvitae are subject to spider mite problems and can suffer from root rot if the soil drainage is poor. Either problem can result in the symptoms you describe. Before doing anything I'd take a sample of both the dead and dying foliage to your local Cooperative Extension office or well staffed nursery so a professional can diagnose the problem first-hand. They can also suggest control strategies. Hope your plants will be okay!
Q&A Library Searching Tips