Answer: Thuja is typically not affected by insect and disease problems but they do prefer moist soils. They can dieback rapidly if drought stressed but can exhibit the same symptoms if the soil retains too much water. You might try digging around the root system of one of your thujas. Root rot will be quite apparent; mushy black roots, rotten bark and an odor. Drought stress will produce rusty colored roots.
Hope you can determine the cause of decline in your thuja.
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