|My Chinese Lilac's leaves are turning brown|
Something most likely is wrong in the roots. Physican damage to the roots, drought, root death from soggy soil conditions or burn from excessive fertilizer applications are all possible causes. There is also the possibility that a disease problem such as powdery mildew did enough damage to cause the foliage to brown but if so you should have seen some white on the leaves earlier on.
I suggest that you take a sample from the plant along with a photo of the plant in its growing location (if possible) to your County Extension Office to see if they can help diagnose the problem and recommend an appropriate course of action.
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