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Lilac Diseases: Blights, Mildew, and Witches' Broom

Disease Reference Table

Disease	Symptoms	Pathogen/Cause	Management
Ascochyta Blight	Spring: current year's shoots and flower stalks girdled, wilted, brown. Base of dead area tan/gray, shriveled. Wet weather: dark gray pimple-like fruiting structures on dead tissue. Summer/autumn: olive green round leaf spots turn tan, indefinite edges; fruiting structures on upper spot surface.	Ascochyta syringae	Prune infected tissue. Avoid overhead irrigation. Apply fungicide to protect healthy shoots.
Bacterial Blight	Leaves turn completely brown/black, remain attached. Shoots girdled and killed. Flower buds blackened; clusters limp and brown. (See shoot blight below.)	Pseudomonas syringae	Avoid overhead watering in spring. Prune infected branches well below diseased tissue. Disinfest shears between cuts. Apply bactericide to protect healthy shoots.
Powdery Mildew	Dry white fungal growth on leaf surfaces; leaves become distorted.	Microsphaera syringae	Apply fungicide as soon as mildew observed.
Shoot Blight	Shoots killed extensively (4–5 ft), turn very black. Root sprouts at base killed and blackened. (See bacterial blight above.)	Phytophthora cactorum	Remove infected plant; do not replace with Phytophthora-susceptible plants. Avoid overhead watering of healthy plants.
Witches'-Broom	Short, thin twigs/branches from one stem area form dense clusters. Leaves distorted, small, yellow. Twigs abnormally upright, retain green leaves too long in autumn, die back in winter.	Phytoplasmas	Prune affected branches. Remove severely infected plants.

Powdery mildew.

Plant Disease: Lilac Bacterial Blight: Prune and Spray Copper; Lilac Buds Wilting, Gray Bark Spots: Multiple Pests and Diseases; Plant Diseases 101: What to Do When Your Plant Looks Sick; Plant Virus Diseases: Symptoms, Transmission, and Control Strategies; Palm Disease Guide: Nutritional, Environmental, and Fungal Disorders; Pansy Disease Management: Anthracnose, Leaf Spots, and Root Rot Control; Cotoneaster Disease Management: Botryosphaeria Canker and Fire Blight Control; Elm Tree Diseases: Symptoms, Causes, and Management Strategies; Fire Blight: Symptoms, Disease Cycle, and Management for Rosaceous Plants; Lilac Diseases: Blights, Mildew, and Witches' Broom

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