brown discoloration of lilac leaves
The leaves on our lilac plant have an unusual distribution of brown. It affects the periphery of the leaves and in between the veins. The veins remain green for about 3/16 of an inch all around the veins. Is this due to lack of water?
Baltimore County Maryland
We have seen multiple examples of very stressed lilacs this year -- leaf spots, browning, and early defoliation. Yes, water deficiency could be an issue if it has been very dry in your area and especially if it is a newly planted lilac that has not been watered regularly to help it establish the root system. Plants will shed leaves early that are either too damaged to function well or for which the plant does not have enough moisture to retain.
Late spring frosts this year damaged tender leaf tissues that had already broken dormancy, facilitating infection from bacterial leaf pathogens. These diseases don't always cause symptoms of infection until later in the season, when summer heat stress worsens for the plant. (https://extension.umd.edu/hgic/topics/bacterial-blights-lilac)
Powdery mildew is relatively common on lilacs and can cause leaf damage beyond just a white/gray coating on the leaves; the damaged leaf portions can "burn" and crisp after summer heat and drought stress them.
These are some possibilities for the browning and curling you are seeing.
Monitor your plant for watering needs during the autumn months, soaking well but only as needed when the top several inches of the soil has dried somewhat. Refer to our guidelines on watering and how to check the soil moisture. https://extension.umd.edu/hgic/topics/watering-trees-and-shrubs
We have had autumn droughts in the past - just last year in fact - so continuing to monitor your plant's moisture until the frosts arrive would be beneficial.