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Peggy Sellers, Master Gardener State Coordinator
The first symptoms of this bacterial disease, caused by Pseudomonas syringae pv syringae, usually appear on the leaves as irregular to circular dark brown spots with yellowish halos. Infected parts may die rapidly and become dark brown to nearly black. Infected leaves may become distorted. This disease can kill leaves, flower clusters, and shoots.
Chinese, Japanese, Persian, and common lilacs are susceptible to bacterial blight. The new succulent leaves and shoots are most susceptible. Infection is favored by mild, wet weather, during which time the bacteria multiply quickly and are spread by splashing rain.
Management of bacterial blight of lilac is best achieved by cultural practices. Dense clumps of lilacs should be thinned to promote air circulation. Infected shoots should be cut out and destroyed as soon as they are noticed, preferably when the plants are dry, to avoid spreading the bacteria. When pruning out infected shoots, disinfest the shears by dipping them in 70 percent denatured alcohol or Lysol after each cut.
Plant and Pest Digital Library and Digitally Assisted Diagnosis, Purdue University, West Lafayette, Indiana.