A lilac bush makes an ornamental natural decoration for a garden or yard, blooming with bright flowers and lush green leaves through most of the spring, summer and fall. If the leaves of your lilac are curling or distorting, you may be facing either a disease or an insect infestation.
Powdery mildew will rarely cause a lilac bush to die but causes significant surface damage and defoliation. Curling leaves are an early symptom followed by white, mold-like growth covering the leaves and flowers. Eventually, the leaves turn brown and black and fall off the bush. High heat and humidity, overwatering, lack of sunlight and closely spaced plants all lead to the development and spread of powdery mildew among lilacs.
Trimming your lilac bush regularly ensures enough sunlight and air penetrates to all of the leaves and flowers. Cut back areas where branches rub together. Mix organic soil amendments, such as compost or manure, into the surrounding soil to improve drainage. If necessary, transplant the bushes to a sunnier area or farther apart for increased light and airflow. Some fungicides; including chlorothalonil, copper and sulfur; help combat powdery mildew.
Aphid infestations are common on many types of plants including lilac. Aphids live and feed on the underside of lilac leaves, causing them to curl or producing small holes in the leaves. Like powdery mildew, an aphid infestation rarely kills the plant but causes the leaves to curl, die and fall.
Some birds will feed on aphids. Other natural predators include spiders, beetles and ladybugs. Treat aphids with horticultural oils or pesticides. Different areas provide different chemicals for killing garden insects, while other chemicals are banned in some locations, so consult a local garden center, nursery or extension service for details regarding your aphid-control options.