Lilac Tree Insects

Powdery mildew

• Powdery mildew is a common disease that can transform a beautiful bush into an unsightly plant by the late summer. Most varieties of lilac are susceptible to the disease, which is carried by a number of woody plants. The spores are released during wet weather in the spring, germinate and spread to neighboring plants. Favorable conditions include humidity, over-fertilization, too much watering and overcast weather.

  Powdery mildew shows itself on lilacs with felt-like gray or white patches on the leaves. Eventually, the whole leaf becomes covered with the fungus. Brown, yellow or black spots will also appear. The leaves will begin to drop earlier than they should in the fall. This doesn't affect the overall health of the lilac, but ruins the appearance in your landscape.

Lilac Leafminer

• The lilac leafminer is an insect that feeds on leaves, injuring the bushes. The moths feed for their entire larval period inside the leaf. While it doesn't threaten the plants, the insects cause cosmetic changes. The spots affected by the miners dry out and die. Outbreaks usually don't last long but can be treated with insecticides. There are contact insecticide sprays that can be applied to leaves when the eggs are laid or neonicotinoid insecticides that can be spread in the soil for the roots to suck in.

Lilac Borer

• Lilac borers affect shrubs and trees throughout Canada and the eastern United States, as far west as Colorado and Texas. The insects burrow into the main stems of the plant, resulting in severe wilting during hot weather. Infested lilacs look unhealthy, with cracked and swollen stems that break off easily. You can also visibly see holes in the wood. Females lay eggs in late summer or early fall, overwinter, then tunnel into the wood the next spring and summer. A residual pesticide can be sprayed in Auguest on plants that are prone to lilac borer.

Other Prevention/Treatment

• Pruning will prevent the chance of powdery mildew and other infestations. Removing branches will open air circulation, which helps stave off infestation. Trim branches after flowering, just as next year's blooms are being produced. Cut down long, leggy stalks at the base of the plant. Remove diseased, damaged or weak stems such as those caused by lilac borer. Prune young shoots to half their length, cutting them where they join other branches.

  Damp areas with dense lilac bushes are most prone to insects and powdery mildew. It's best to plant the trees in a sunny spot with good air and water drainage. Use a proper fertilization method, following the instructions from the manufacturer. Too much fertilizer can cause excess growth, which attracts infestation.