Some researchers agree with this recommendation and some do not. Removing seed capsules also improves the plant's appearance. Pests Weeds can be a problem around lilacs but can be controlled by hand weeding or by mulching. Borers , insects that commonly burrow into lilac stems, cause leaves to wilt by inhibiting water movement. Severe borer injury can also cause the stems to break off.
Some lilac species are more resistant to borers than others.
Appropriate pesticides applied at the proper time can reduce or eliminate borers. They injure plants by sucking sap from stems, weakening the plants. Scales can be controlled with horticulture oils. Powdery Mildew is a disease affecting lilacs statewide, infesting the leaves and leaving a whitish, dusty, unsightly appearance. Because the disease normally appears at the end of the growing season, it is seldom permanently damaging.