Lilacs such as the Japanese tree lilac are members of the Oleaceae family. These woody ornamentals can be used as a large shrub or a small tree and produce lovely fragrant flowers. Often used as accent shrubs or borders, these lilacs are generally healthy. However, they are susceptible to a damaging wilt disease that can cause sudden wilting.
Verticillium wilt is a common disease in woody ornamentals such as lilacs. Some other plants susceptible to this disease include redbud, smoketree, maple and ash. The Verticillium dahliae fungus, which lives in the soil, causes this disease. The symptoms associated with verticillium wilt include premature fall colouration of foliage, sudden wilting and premature leaf drop. Some lilacs may develop yellow leaves and stunted growth. Eventually, the entire shrub dies. Many home gardeners confuse the symptoms associated with verticillium wilt with environmental stresses. When verticillium wilt is the cause of plant death, the internal wood may be streaked with a green or brown.
No fungicides are available for the treatment of verticillium wilt. Lilacs that show severe signs of disease cannot be saved and should be removed. Avoid planting other susceptible species in the same area, as the fungus remains in the soil for many years. Do not use trees or shrubs with verticillium wilt for mulching or compost. Frequent and regular watering may help reduce the symptoms of verticillium wilt in lilacs and other plants.
Lilac ash borers are common pests of lilacs, privets and ash. The adult form of this pest is a clear-winged moth that lays its eggs at the base of host trees. After hatching, the larvae tunnel into the plant through the bark and feed on the internal wood of the tree. The symptoms associated with the lilac borer include wilting, dieback and reduced vigour. Lilac borer damage usually begins at the base of trees, but can be found as far as 25 feet from the ground. Lilacs infested with this pest may have boring holes on the trunk, which may ooze sap. Infestations are most severe when lilacs are under stress.
Pheromone traps may help home gardeners determine when adult moths are present and ready to lay eggs. Preventive sprays applied to the trunk of susceptible plant species may help reduce infestations. Pruning older branches from susceptible trees may also help reduce infestations of lilac borers because this pest prefers to infest older branches. Trunk sprays with the active ingredients bifenthrin or permethrin may reduce infestations, but imidacloprid will not kill the pest.