Bacterial blight of lilac is caused by the bacterium Pseudomonas syringae pv. syringae. Several samples showing the presence of this disease have been recently received in the Plant Disease Clinic. Initial symptoms include brown, water-soaked spots on leaves. These spots usually enlarge and coalesce, often causing leaves to become misshapened. Eventually leaves may be killed. When the infection spreads around a twig, it becomes girdled and dies. This phase of the disease is evident as young new shoots develop in the spring. Shoots turn a black color, droop over, and die.
Control consists of pruning out blighted twigs as soon as they occur. Pruners should be dipped in alcohol or a 10% bleach solution between cuts to prevent spreading the disease. Cut several inches below the margin between healthy and diseased tissue. Prune in dry weather only, and be sure to remove and destroy all infected tissue. Where possible, thin plants to provide good air circulation. The use of a copper-based product such as Bordeaux mixture may aid in disease control. Applications should be started when new growth appears in the spring. Follow the label instructions of the product purchased for specific rates and timing.
Lilac blight can be confused with frost injury to newly emerging tissue. Cold temperatures may cause blackening of leaf tissue, with new shoots turning a black color and drooping over. However, the brownish, watersoaked spots characteristic of the bacterial disease are not evident. Also, with cold temperature injury, the blackened leaf tissue may be more prominent on the margins of leaves.
When diagnosing a lilac in poor vigor, also examine the stems and trunk for evidence of any mechanical injury, (such as by borers, animals, or mowing equipment).
This article originally appeared in the June 7, 1996 issue, p. 93.