Informational table showing disease name, symptoms, pathogen/cause, and management of Lilac diseases. Disease Symptoms Pathogen/Cause Management Ascochyta blight In the spring, the current year’s shoots and flower stalks are girdled, wilted, and brown. The base of the dead area is tan to gray and shriveled. In wet weather, dark gray pimple-like fungal fruiting structures dot the dead tissue. In summer and autumn, olive green round leaf spots turn tan and have indefinite edges. Fungal fruiting structures dot the upper surface of the spots. Ascochyta syringae Prune infected tissue. Avoid overhead irrigation. Apply a fungicide to protect healthy shoots. Bacterial blight Leaves turn completely brown to black and remain attached to the branch. Shoots are girdled and killed. Flower buds are blackened while flower clusters become limp and brown. See shoot blight below. Pseudomonas syringae Avoid overhead watering in the spring. Prune infected branches, cutting well below the diseased tissue. Disinfest the pruning shears between cuts. Apply a bactericide to protect healthy shoots. Powdery mildew Dry white fungal growth develops on the surface of leaves. Leaves become distorted. Microsphaera syringae Apply a fungicide as soon as mildew is observed. Shoot blight Shoots are killed extensively, up to 4 to 5 feet, and turn very black. Root sprouts at the base of the plant are killed and blackened. See bacterial blight above Phytophthora cactorum Remove the infected plant; do not replace it with plants susceptible to Phytophthora. Avoid overhead watering of healthy plants. Witches’-broom Short, thin twigs and branches originating from one area of the stem form dense clusters. Leaves may be distorted, small, and yellow. Twigs forming the brooms are abnormally upright and often retain green leaves too long in the autumn and die back in winter. Phytoplasmas Prune affected branches. Remove severely infected plants.

Powdery mildew.