Answer: Based on your description I am not certain what has affected your lilacs; these particular lilacs are usually pretty much trouble-free with healthy foliage. You might want to check with your local county extension to obtain a specific diagnosis and based on knowing that, you would know better what to expect next year. If it is something that requires treatment, they should be able to tell you what to use and how/when is best to apply it. In the meantime, clean up and dispose of all of the old foliage this fall. Also consider replacing the mulch beneath them in late fall. This should go a long way toward avoiding carryover to next season. Good luck with your lilacs!
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