Answer: How strange! It must be disappointing to have your shrubs decline in this manner. It's almost like you can predict which one will keel over next. Usually browning tops indicate a root problem. Overly soggy soils can really affect thuja's so while deep watering is important, it's also important that the soil drain quickly. Perhaps the soil is remaining too soggy between waterings. If so, root rot can compromise your plant's health and it could spread to nearby thuja's. The only other thing that might cause foliage browning and spread from one plant to another is an insect infestation. Check the inner branches and foliage for signs of spider mites. They are tiny so your first clue will be webbing on the stems of the foliage. Spider mites are about the size of a pepper grain. You can take an index card or a sheet of white paper, hold it toward the interior of the plant and sharply tap the branches. Debris will fall and if what looks like pepper grains begins to move, your plants have spider mites. If you closely inspect your plants and can find no evidence of insects, you might want to take a sample of the affected plant parts to your local cooperative extension office for positive diagnosis. They can determine exactly what's wrong and provide control strategies.
Best wishes with your plants!
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