Bacterial blight of lilac
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Bacterial blight of lilac, caused by the bacterium Pseudomonas syringae pv. syringae, can be a potentially serious disease of lilac. Samples have recently been received from nurseries and landscapes with bacterial blight of lilac. Initially, small water-soaked spots may be seen on the leaves, later expanding and coalescing to larger blighted areas. The youngest shoots are most susceptible, and if severely affected will appear as black, crispy tissue. Shoots may be girdled by infections. Succulent shoots often wilt, but woody tissues remain erect. P. syringae pv. syringae actually produces toxins that are detrimental to the lilac cells, contributing to its pathogenicity. Several types of lilac can be affected including Chinese, Japanese, Persian and common lilac.
Lilac blight may be easily confused with frost injury, which was also present on lilacs this spring. Now we are past the time period when frost damage is likely (hopefully). But an interesting thing about this Pseudomonas is that at temperatures slightly below freezing, the bacterium acts as a nucleus to form ice crystals, thereby initiating more damage than either frost or Pseudomonas singly. As a result plants infected with Pseudomonas tend to be more susceptible to freezing.
Copper fungicides are known to be somewhat inhibitory to Pseudomonas bacteria, but managers of lilac will be very disappointed if fungicides are used as the sole or primary focus of control. Blighted shoots should be pruned well into "green" cambial tissue, make pruning cuts at least six to eight inches below any visible signs of infection. This will also stimulate lateral bud development for new shoots. Never prune when leaves are wet or during wet weather. Pruners should be cleaned with a disinfectant between cuts. The tissue that is pruned out or crumbles as leaf litter is still infectious and should be removed and destroyed to prevent further disease spread. If practical, eliminate overhead irrigation, which spreads millions of the bacterial cells (literally) to healthy tissue. Good plant spacing will help reduce short distance spread and promote more rapid drying of the foliage.