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. No chemical control is listed.
Phytophthora blight kills stems to the ground. The leaves turn black and shoots have brown lesions on them. No chemical control is listed.
Leaf blotch causes zoned, brown spots. The infected area drops out, leaving a hole in the leaf. No chemical control is listed.
Many fungi cause leaf spots. If needed, use basic copper sulfate.
Powdery mildew coats the leaves with white powder. During wet weather, lilacs mildew easily. Mildew is especially severe on shade-grown plants. Spray with benomyl or sulfur. Ignore late season infections.
Verticillium wilt causes wilting and premature leaf drop. The disease may kill one, several or all the branches. There is no chemical control. Try fertilizing with high nitrogen fertilizer.
Bacterial crown gall causes round, warty galls on the stems near the soil line. There is no chemical control. Remove infected plants and do not replant in the same spot.
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