Answer: Arborvitaes are subject to several fungal diseases in the Pacific Northwest and it's difficult to diagnose the problem with yours without actually inspecting the plant. Rust colored spots indicate dead foliage. If the spots are lower down on the plant, could it be dog urine? Leaf miners can attack arborvitae and turn the foliage yellowish and then brown so that's a possibility, too. For absolute diagnosis and control options, take samples of the affected and healthy foliage to your local cooperative extension office. Helpful folks there can diagnose the problems with your arborvitaes. You didn't indicate whether you're in western or eastern Oregon so I can't direct you to a local office. However, the following website will allow you to enter your county and find the nearest cooperative extension office:
Best wishes with your arborvitaes!
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