Lilac borers are insects whose creamy white larvae tunnel into branches. Wilted leaves and holes in the affected branches point to an infestation. Any affected branches should be removed and destroyed at the first sign of symptoms.
Oyster-shell scale. These are small sucking insects about 3 mm long with a brownish shell shaped like an oyster shell. A serious infestation can kill the affected stems. Horticultural oil spray applied during dormancy can help eliminate these insects, but it is often best to remove and destroy any severely affected branches.
Lilac leaf miners are moths whose larvae burrow and tunnel into the leaves. The damage they cause is fairly cosmetic. Remove affected leaves before the larvae fall to the ground to minimize subsequent infestations.
Powdery mildew. The symptoms of this fungal disease take the form of a white down on the leaves that generally appears when days are hot and humid. This disease does little actual damage to the plant.
Various forms of leaf spot, caused by bacteria or viruses, may affect lilacs. Remove diseased branches, taking care to disinfect your tools.
Witches’-broom, caused by a micro-organism, results in densely clustered abnormal growth of shoots. Lilacs in the Syringa x prestoniae group are the most vulnerable to this disease. Remove all affected parts, taking care to disinfect your tools between cuts.
Lilac bacterial blight is a disease that causes tissues to burn and young shoots to weaken and become susceptible to frost damage. The terminal shoots become dry or scorched looking. The leaves may be deformed, with brownish blotches. All affected parts must be removed by pruning 60 cm into healthy wood, remembering to disinfect your tools between each cut.
Meadow voles are small mammals that can do considerable damage by gnawing lilac bark. A good way to prevent such damage is by installing a physical barrier, such as a white, perforated wrap.