Lilac bushes with curled leaves are suffering from a serious health problem that can affect continued flower production and the longevity of the plant. Curled leaves are less able to photosynthesise, which weakens the entire lilac bush and makes it more susceptible to further damage from pests and diseases. Treat the lilac bush to keep it from dying.
Several problems can cause lilac leaves to curl. The leafminer, a small insect that tunnels inside leaves, causes the leaf to curl up around it as it weakens the structure of the leaf. Severe infestations cause the entire lilac bush to exhibit curled leaves. Additionally, fungal diseases such as powdery mildew cause leaves to curl as the leaves are wounded and the plant starts to die.
Applying pesticides to control leafminers is important for protecting the leaves of the lilac bush. It is also important to apply dormant horticultural oils in the early spring to kill any remaining eggs from the previous season and prevent further infestations from occurring. Protecting the lilac is key to keeping its leaves healthy. Although leafminer damage is primarily a cosmetic problem, it does make the lilac more susceptible to other pest and disease problems. Pesticides containing pyrethroids are effective.
Prune lilacs that show visible signs of leaf curling, as well as other symptoms of diseases, including wilt, brown or black spots on the leaves and leaf drop. Removing severely damaged branches helps slow the spread of the disease and encourages new lilac growth to emerge, which helps restore the plant's health. Take any fallen leaves, branches and flowers out of the garden to prevent them from hosting diseases and pests that affect the lilac.
Apply fungicides to lilacs that have curled leaves; fungal diseases such as powdery mildew spread until they cover the entire plant and sometimes lead to the death of the lilac. Fungicides formulated to control powdery mildew help kill the fungal spores and stop the spread of the disease. In addition to pruning the lilac, killing the fungi helps protect the shrub or tree against any further damage and encourages the plant to start producing new growth.