This plant disease is caused by a bacterium (Pseudomonas syringae) that may seriously damage lilacs during cool, wet weather. The bacteria overwinter in lilac buds, infected twigs, and plant debris and on other hosts. They are spread by wind, rain, and splashing water. Frost while buds are swelling or shoots are just beginning growth favors disease development. The bacteria cause spots on leaves that are olive green at first and later turn brown surrounded by yellow. If wet weather persists, the bacteria spread through the tissue, forming blotches. Young plant parts are more severely affected. Leaves and young shoots blacken rapidly and die. Leaves die on older stems within the infected area. Flowers often become limp and blighted.

Lilacs may fail to bloom for several reasons. 1. Cold injury: Extreme winter temperatures or late spring cold snaps can kill lilac flower buds, which form during the late summer or fall. 2. Improper pruning: Because lilacs produce flower buds in the summer, pruning in the winter or spring will remove these potential flowers. 3. Too much shade: Lilacs growing in deep shade may fail to form flower buds. 4. Heavy pruning or transplanting: Lilacs that have been pruned heavily or recently transplanted will often wait a year or more to bloom.

Podosesia syringae The lilac borer, also called the ash borer, is the larva of a brownish, clear-winged moth that resembles a wasp. Moths fly around the plant in late spring. The moths lay their eggs in cracks or bark wounds at the base of the stems. The cream-colored larvae bore into the wood, feeding on sapwood and heartwood. The stems become swollen and may break where the larvae are feeding. Their feeding also cuts off the flow of nutrients and water through the stems, causing the shoots to wilt and die. The larvae spend the winter in the stems. In spring, they feed for a few weeks before maturing into moths. Several other borers may infest lilac.

Caloptilia syringella The lilac leaf miner is the larva of a small brown moth that infests both lilac and privet. In late spring, moths emerge and deposit their eggs on the lower surfaces of leaves. The emerging larvae enter the leaves and mine within the tissue between the upper and lower surfaces. About the time the mines turn brown, the worms crawl to the leaf tips and curl the leaves around themselves. They feed for several weeks inside the leaves and then drop to the ground to pupate. The adults that emerge in late summer lay the eggs of a second generation, which survives the winter as pupae in plant litter.

This common plant disease is caused by a fungus (Microsphaera alni) that thrives in both humid and dry weather. The powdery patches consist of fungal strands and spores. The fungus saps plant nutrients, causing yellowing and sometimes the death of the leaf. Since this mildew attacks many different kinds of trees and shrubs, the fungus from a diseased plant may infect other plants in the garden.

Several species of scales infest lilacs. They lay their eggs on the bark, and in early to late spring the young scales, called crawlers, settle on the trunk, branches, and twigs. The small (1/10 inch), soft-bodied young feed by sucking sap from the plant. The legs usually atrophy, and a hard crusty shell develops over the body. Mature female scales lay their eggs underneath their shells. In warmer climates, there may be several generations a year. An uncontrolled infestation of scales may kill the plant after two or three seasons.