Lilacs are susceptible to several leaf problems.
The lilac genus consists of several species of flowering, temperate-climate shrubs that produce vibrant blossoms. Favored for their cold hardiness, lilacs are free of most major problems. However, disease and insect damage can threaten the vitality of lilac. Identification, prevention and early control are vital for minimizing the damage from these lilac pests.
Lilac Leafminer
Lilac leafminers are the larvae of a small brown moth. The larvae tunnel and feed within leaf surfaces, causing extensive foliage damage. Symptoms of infestation consist of irregular brown lines on the upper and underside of leaves, leaf curl and leaf distortion. Remove and destroy fallen and attached leaves to reduce the spread of the larvae. Chemical insecticides are an effective method of prevention and control for severe infections. However, broad-based insecticides can also kill predatory insects, which can increase the severity of leafminer infestations.
Powdery Mildew
Powdery mildew is a common fungal disease of nearly all leaf-bearing shrubs, trees and plants, including lilac. The Microsphaera penicillata fungus causes powdery mildew on lilac. Symptoms occur midsummer and consist of white felt-like powder on leaf surfaces, leaf drop and leaf distortion. While the disease may appear serious, mild to moderate powdery mildew infections rarely cause more than cosmetic damage. The fungus favors warm weather and shady growing locations for infection. However, unlike other fungal diseases, it does not require free moisture on leaf surfaces for infection. Fungicides are effective for controlling powdery mildew infections.
Lilac Blight
Lilac blight or bacterial blight is a serious disease of lilac, caused by the Pseudomonas syringae bacterium. Symptoms of infection consist of shoot dieback, water-soaked dark green spots on leaf surfaces, dark brown blossoms and leaf death. The bacterium favors tree wounds and openings, cool weather conditions and free moisture for infection. The University of Wisconsin Extension recommends pruning and burning infected branches and disinfecting pruning equipment with a 70 percent alcohol solution to reduce the spread of the disease. However, copper-based chemical control is an effective method for controlling severe blight infections.
Root Weevil
The lilac root weevil lives in the soil surrounding the plant and feeds on leaf surfaces at night. The weevil is small, measuring about 4/10 inch long. Symptoms appear during the late spring and early summer and consist of minor leaf notches. Foliage damage from root weevil infections is mostly cosmetic and rarely causes permanent damage to the lilac. The Colorado State University Extension recommends parasitic soil nematodes in the genus Heterorhabditis to control root weevils. However, insecticidal control is effective for controlling severe infestations that reduce the vitality of the shrub.
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