Answer: A variety of factors can cause this type of symptom ranging from stress to over fertilization to root damage to soil compaction to accidental chemical or herbicide exposure. But based on your geographic location and your description, although I am not certain what is happening, it might be related to drought stress. If the soil is dry, try a slow deep watering that soaks down into the soil at least six to eight inches. Dig down after watering to make sure it has reached that depth. If you are not using a mulch around the lilac, apply an organic mulch two to three inches thick after watering. Spread the mulch out in a flat layer over the root area and keep it a couple of inches away from the stem/trunk of the plant. This will help keep the soil damp.
If you do not think it would be related to dry soil, you might want to consult with your local county extension to try to obtain a more specific diagnosis. I hope this helps you trouble shoot.
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