Lilacs prefer full sun and well-drained soil for growth. Although they require several years to become established and produce blooms, established lilac shrubs are long-lived. Some survive for centuries on abandoned homesteads. They prefer slightly acidic soil, but tolerate neutral to slightly alkaline soil. These hardy shrubs require little care other than pruning and occasional supplemental watering in periods of drought. Many are disease-resistant, but may fall prey to occasional disease.
Often referred to as shoot blight or blossom blight caused by the Pseudomonas syringae bacterium, bacterial blight is one of the most common ailments seen in lilacs. This disease results in brown spots on the stems and foliage of lilacs, causing leaves and stems to die back from the tips of the leaves to the stem. Treatment consists of proper pruning to increase air circulation, avoiding wetting foliage when watering and providing proper fertilization.
Powdery mildew often affects lilacs late in the season and leaves a white or gray powdery substance on foliage. It primarily affects leaves and growing tips, causing yellowing of foliage and often resulting in reduced growth rate and vigor. Treat with fungicide labeled for powdery mildew. Remove any debris under the shrubs in the fall to prevent spreading the disease. Proper spacing, yearly pruning to improve good air circulation and avoiding getting foliage wet when watering assists in preventing powdery mildew.
Sooty mold, characterized by a layer of black covering the foliage and stems, results from mold spores growing in honeydew. Honeydew is a secretion made by aphids and other insect pests feeding on the plant tissue. Although sooty mold can be washed off lilacs, unless the insects are removed it will likely return. Control insects with the appropriate insecticide, following the manufacturer's instructions and application rate.
Leaf scorch appears as yellow or dark areas between the main vein of the leaf and the margin. Foliage may drop in late summer, and tips of new growth may die. Poor soil, lack of nutrients, drought and overcrowded roots all cause leaf scorch. Fertilize lilacs in early spring by spreading fertilizer over the area beneath the canopy of the tree. Water thoroughly to saturate the soil to a depth of 6 inches once a week during dry spells. Prune to maintain shape and reduce the total foliage to reduce stress on overcrowded root systems.