The older, inner leaves of arborvitae (Thuja) naturally turn brown and drop in the autumn (similar to the loss of leaves on deciduous trees). This shouldn't cause alarm, and does not indicate disease. If the tips of the branches are turning brown, look for infestations of spider mites (they leave tiny webbing between the junctions of the leaves and stems), or consider cultural problems such as over- or under-watering. Sometimes the leaf tips will turn brown because they've been exposed to sudden temperature changes. If your arborvitae has just recently been planted, it may be going through an adjustment period. Give it about an inch of water every week, and prune off the brown tips to encourage healthy new green growth. Good luck with your arborvitaes.
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