Lilacs are vigorous perennial bushes with lush, fragrant spring blooms. Lilacs are generally easy to grow and have relatively few major problems. Regular maintenance and careful pruning are the first line of defense in avoiding problems with lilac bushes. Healthy plants are better equipped to fight off attacks from insects and damage from disease. However, even the hardiest plants sometimes develop problems, and lilacs are no exception.
Lilac borers are caterpillars that feed on the soft tissue in the stems and branches of the bush. The leaves of infected plants wilt, turn brown, and eventually die. The branches also wilt and break off easily.
Leafminers are the larvae of moths that lay their eggs on the undersides of the lilac leaves. They appear as dried brown blotches on infected foliage. Leafminers may appear any time during the growing season.
Scale is soft-bodied insects that suck sap from plant tissue, weakening them and making them susceptible to further damage from diseases and insect attack. Scale appears as small bumps on plant stems and branches.
Prevention/Solution for Insects
Carefully prune infected branches, using sharp tools to make clean cuts. Remove damaged leaves and stems and dispose of in the garbage. Do not place in your compost as insect larvae may survive the winter to mature into adult moths.
If the insect problem is severe, spray the stems with an insecticidal soap. Apply the spray from late spring to early summer.
In the fall, rake up and dispose of all fallen foliage and debris under the bush to eliminate shelter for insects during the colder months.
Prevent scale infestations by applying dormant oil spray in the spring before the leaves are fully open. The oil smothers the insects.
Powdery mildew is a fungal disease common to lilacs. It appears as dusty white patches on infected foliage. If powdery mildew shows up late in the growing season, it is usually more unsightly than it is damaging to the health of the plant. However, infestations that occur before midsummer can inflict stress and damage on the plant and should be treated.
Prevention/Solution for Mildew
If powdery mildew is detected early, spray with an organic fungicide. In the fall, trim off infected leaves, and clean up any that have fallen on the ground. Put infected foliage in the garbage. Do not add to the compost pile.
Choose lilac varieties that are resistant to powdery mildew.
Plant lilacs in sun and provide good air circulation. Powdery mildew thrives in humid conditions.
Tips for Lush Blooms
It is common for lilacs to bloom more profusely every other year. To encourage lush blooms annually, cut off the flowers after they fade. Take care not to cut spent blooms too close to the base of the stem, where the next year's buds are forming.
After the lilac is done blooming, remove one-third of the older stalks every year to ensure that you never have a year without blooms. Prune the stem down to a few inches about the ground.
Use only products that are specially formulated for application on lilacs.
Follow label instructions carefully when applying insecticides and fungicides.