Lilacs (Syringa vulgaris) add springtime fragrance to yards and landscapes. Lilac bushes grow 8 to 15 feet tall and 6 to 12 feet wide, and the upright flower clusters bloom in a variety of colours, including purple, pink and white. These shrubs prefer moist, well-drained, alkaline soil, in an area that receives full sun exposure. Proper growing conditions help prevent fungal and bacterial diseases.
The Ascochyta syringae fungus causes this blight during periods of wet weather. Symptoms include brown, wilted stalks and green or brown, irregularly shaped leaf spots. During wet weather, an infected lilac bush develops tiny dark grey bumps, or spore structures, on dead surfaces. To prevent this disease, remove and destroy infected plant material and avoid overhead watering. Effective chemical controls include thiophonate methyl and chlorothalonil.
The Pseudomonas syringae bacteria overwinters on infected plant material, and spreads during windy, wet weather. The first symptom of bacterial blight is often water-soaked spots on lilac leaves and stems. Lesions form on the stems, causing the stem to bend and wilt. The spots on the leaves turn brown, followed by the leaves, buds and blossoms turning brown or black. The discoloured, wilted plant parts wither and die, but do not drop off the plant. To control this fungus, prune back and destroy infected stems, and avoid overhead watering. Sterilise your gardening tools after pruning, and spray new growth with mancozeb and copper.
Also known as grey mould, Botrytis cinerea fungus produces a grey mycelium web on lilac leaves, stems, buds and flowers. Brown lesions form in the centres of the flower petals. According to the University of Illinois Extension, the fungus is active in rainy weather at temperatures between 0 and 28.9 degrees Celsius. Water drops or a decrease in humidity release the spores; wind carries the spores from dead plant material to weak or injured plant tissue.
Phytophthora Shoot Blight
The Phytophthora cactorum fungus kills injured or stressed plants. Lilac shoots dry out, turn brown and die. The University of Wisconsin Cooperative Extension notes that Phytophthora fungus increases a plant's susceptibility to frost damage. Spore cases develop in the dead tissue and spread to non-infected areas during wet weather.
Powdery mildew occurs during periods of hot, humid weather. The Microsphaera penicillata fungus covers lilac leaves with a white powdery substance. Although the disease does not harm the plant, it causes leaf distortion. To prevent the spread of the disease, plant your lilac bush in an area with plenty of air circulation. To control powdery mildew, the Pennsylvania State University Cooperative Extension recommends an application of paraffinic oil to infected plant surfaces. Other effective control methods include horticultural oil, thiophanate-methyl and triadimefon.