Sick cedar

Hi!
I bought a small Emerald Cedar (10" pot, plants about 12" tall) that I keep in my office. I've had it a few months and it's been getting bigger, lots of new green growing. Lately the old growth has been getting darker and turning grey and brown and getting very brittle. It's spreading through the plant and I can't find anything that describes the problem.
Plant gets good light and I water occasionally, so I know it's not drying out or overwatered.
Any suggestions?
Thanks!
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