There are a number of possibilities for your ‘Emerald’ eastern white cedar (Thuja occidentalis) problem. A common cause of foliar browning in ‘Emerald’ cedars is due to root rot or root damage. We have had a wetter than normal season. If your garden is on clay soil and drainage is slow, the heavier rains this season might have caused problems at the root level. Other root-related issues include compaction of soil or mechanical damage. Thuja roots are relatively shallow/close to the surface, and at 10-12 years, your cedars’ root systems would be quite extensive – growing much further from the tree than you’d expect. So any new digging, for example, even if not immediately beside the trees might have an impact. Additonally, you should check closely to see if you detect any insects or diseases. Sucking insects such as mites or scale could contribute to the browning you see. This could also be winter or salt burn from the winter season. Once you determine the cause of the browning, then you can better treat it.
This could also be cedar apple rust fungus disease. It can attack cedars planted near apple trees or members of the apple family, including crabapples, hawthorn, Juneberry and mountain ash. These trees serve as “alternate hosts” for a fungus to develop and thrive. See if you can find gall-like growths on the branches. Cut out the galls. Then spray the cedar with a fungicide application to prevent new infestations.
We also recommend if you have eliminated bugs, which are treatable, and the majority of the tree is brown — your cedar may be severely damaged. If most of the foliage is dead, the tree may not be worth saving. Take a picture of the tree and show it to a professional at your nearest nursery — or better yet, take it to your nearest cooperative extension office. An extension employee will confirm whether the tree can be saved.
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