Answer: It's really difficult to diagnose a plant problem without actually seeing it. While thuja can develop spider mite problems, it isn't likely that yours have since they've been in the landscape only 6 months. A more likely cause of the foliage browning is a condition related to the soil or drainage. To be absolutely sure of the the cause of the browning, why not take a sample of the affected foliage to your local Cooperative Extension office? There's a plant problem diagnostic clinic there and trained volunteers will look at the samples under a microscope to determine just why they look abnormal. They'll also take a history of your plant; watering and fertilizing, exposure to sunshine, soil type, etc. Contact the Whatcom County Cooperative Extension at 360-676-6736; 1000 N. Forest St. in Bellingham.
Best wishes with your landscape!
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