Hello Mike,

Thank you for the photos. They are very helpful. From the appearance of your lilac leaves, it is possible that this is herbicide damage. Our modern weed killers have ingredients in them that can volatilize in the heat of the day and drift wherever the wind currents take them, sometimes landing on our ornamental plants and damaging them. Probably the most common cause of herbicide drift comes from the popular lawn fertilizers with weed control. The damage can come from your own use of this type of product or can come from a neighbor’s yard.

If you suspect herbicide damage, remove all damaged leaves. You could check a few of the main stems at the base by taking your fingernail and scraping a small part of the outer layer of bark. If the layer beneath the bark is green, that stem is still alive. Give it adequate water going into fall and water occasionally during the winter when the ground is not frozen.

A fungal infection called powdery mildew can affect lilacs. This usually happens later in the summer when the days are warm and the evenings are cool. If a lilac is being watered by a sprinkler in the evening or during the night, it would be susceptible to powdery mildew. You would have noticed a white powdery substance on the leaves earlier and then they would eventually turn brown, curl up and look similar to the photos. However, I don’t see any evidence of powdery mildew on the leaves in the photos. More info on mildew:

https://planttalk.colostate.edu/topics/insects-diseases/1415-powdery-mildew/

If these two conditions don’t seem like they are possibilities, you are welcome to bring a sample in to the Arapahoe County Extension office for diagnosis. The fee is $7. See further information:

https://arapahoe.extension.colostate.edu/horticulture/plant-insect-help/plant-insect-diagnostic-services/