Lilac Decline –
Identifying the Cause of an Old Disease in Utah
The affected plants will show symptoms on most leaves and shoots.
The symptoms of lilac decline include thickened, twisted leaves with blotchy chlorotic (yellow) areas.
For the last 25 years, lilac decline and death has been observed in Utah, especially in Cache Valley. The problem is most evident in older lilacs, where plants die after a few years of decline. Affected plants have a lack of vigor and new growth is usually limited to less than one inch. Leaves are thickened and smaller than normal. They roll downward and are chlorotic or have yellow-green blotchiness and spots. The discolored areas of the leaves will turn brown, resembling scorch. Shoots will die and the dead leaves remain attached. Lilacs in decline have these symptoms throughout the entire plant.
Many attempts have been made to detect the cause of this decline; no fungi, bacteria or virus pathogens have been found. Then in the fall of 2013, we tested plants for the DNA of phytoplasma and found a species related to the phytoplasma that causes X-disease of peach. We are currently investigating whether this is a single phytoplasma that is responsible for the disease.
Phytoplasmas are bacteria that do not have a cell wall and reside in the phloem of plants. They cannot be cultured in media and can only be identified by comparison of the DNA sequences of the 16S/23S spacer regions. They are transmitted by leafhoppers, psyllids, or by vegetative propagation. Other phytoplasma diseases that occur in Utah include X-disease of cherry and peach, alfalfa dwarf, and pear decline.
Management of phytoplasmas is difficult and infected plants will not recover. The most effective method of control is the development of resistant cultivars. Monitoring plants for leafhoppers and controlling leafhoppers with insecticides can prevent transmission. Once a plant is infected it should be removed promptly to avoid the spread to neighboring plants. Plants should be obtained from reputable nurseries and not from old plantings or neighbors.
-Claudia Nischwitz, Plant Pathologist and
Sherman Thomson, USU Extension Plant Pathologist Emeritus