Answer: Sometimes lilacs are attacked by leafminers. The adult is a fly and she lays eggs just beneath the leaf surface. The eggs hatch and little wormlike larvae tunnel through the tissues between the upper and lower leaf surfaces. They feed as they tunnel and leave brown patterns in the leaves. When they've eaten their fill they make a little hole, spin a web to take them down to the soil surface, and pupate in the soil. In time they hatch into adults and start the whole process over again. Your best defense is to cover the soil with mulch in late summer.
Other browning of the leaves can be caused by a fungus (lilac tip blight). Good garden sanitation is a good defense against this problem. Rake and destroy fallen leaves in the fall; make sure there is adequate air circulation all around the shrub; be sure to plant your lilac in full sunshine, in rich, well draining soil. Once it gets out of the nursery pot and can spread its roots out, it should recover for you. Best wishes with your lilac!
Q&A Library Searching Tips