Answer: Based on your description, I'm not sure what the problem might be. Lilacs are usually free of serious diseases, but they can be affected if they are stressed. Here are the most common problems:.
Bacterial blight is most serious on white flowered varieties. The young shoots develop black stripes or one side of the shoot turns black. Spots develop on the leaves, forming a water-soaked blotch. Young leaves turn black and die quickly. On older shoots, the spots enlarge more slowly. The flowers wilt and darken. The disease is worse when wet weather occurs as the new shoots are developing. Thin plants to increase air circulation. Remove and destroy diseased shoots and avoid excessive nitrogen fertilizer.
Phytophthora blight kills stems to the ground. The leaves turn black and shoots have brown lesions on them.
Leaf blotch causes zoned, brown spots. The infected area drops out, leaving a hole in the leaf.
Many fungi cause leaf spots.
Powdery mildew coats the leaves with white powder. During wet weather, Lilacs mildew easily. Mildew is especially severe on shade-grown plants. Ignore late season infections.
Verticillium wilt causes wilting and premature leaf drop. The disease may kill one, several or all the branches. Try fertilizing regularly to help prevent diseases.
Bacterial crown gall causes round, warty galls on the stems near the soil line. Remove infected plants and do not replant in the same spot.
If these descriptions do not match the symptoms on your lilac, why not take a sample of the affected leaves to your local Cooperative Extension office for a positive diagnosis?
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