Answer: What you describe sounds like bacterial blight. Bacterial blight of lilac, also known as blossom blight or shoot blight, is caused by the bacterium Pseudomonas syringae pv. syringae. Chinese, Japanese, Persian, and common lilac are all affected, with white flowering varieties being the most susceptible. Bacterial blight can easily be mistaken for freezing injury from late spring frosts. The bacterium survives winter in cankers, plant debris, weeds, and soil. The bacterium may also be present on healthy plant tissue year round without causing disease. Bacterial blight can be spread by insects, pruning tools, rain, and wind. Infection starts on the newest growth in the spring, during rainy weather. Early symptoms may include dark brown spots with yellow halos, dark brown blotches starting at the leaf margins and advancing inward, blackened shoots, and black streaking on twigs. Leaf spots may coalesce, resulting in shriveling and death of leaves. Shoots may be girdled, causing death of leaves and blossoms. Stress or injury resulting from poor or improper nutrition, frost damage, or wounding can predispose plants to bacterial infection. Control of this disease can be achieved through the following practices: Prune out and destroy infected shoots 10 to 12 inches below visible infection. Sterilize pruning equipment between cuts with 10% bleach or 70% rubbing alcohol to prevent spreading the bacteria. Properly prune stems to increase air circulation and space plants adequately when planting. Avoid overhead irrigation to minimize splashing the bacteria to healthy leaves and other plants. Properly fertilize, water, and mulch plants. Unlike most other fungicides, copper-based fungicides, such as Bordeaux mixture, can be effective against bacterial problems. If the disease is severe, a copper-based fungicide may be applied in the spring to prevent disease from recurring.
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