Bacterial blight of lilac is caused by the bacterium Pseudomonas syringae. The organism causes brown spots on leaves. These spots may enlarge and coalesce, causing leaves to become misshapen. Eventually leaves may be killed.
When the infection spreads around a twig, the twig becomes girdled and dies. Shoots turn a black color, droop over, and die. This phase of the disease is evident as young shoots develop in the spring. (The symptoms are similar to fire blight on apple.)
Control consists of pruning out blighted twigs as soon as they occur. Pruning cuts should be made several inches below the margin between healthy and diseased tissue. To help prevent spreading the disease, prune in dry weather only and dip pruners in a 10% bleach solution or 70% alcohol between cuts. Also, thin plants to provide good air circulation. A copper based product, such as Bordeaux mixture, applied 2-3 times at 7-10 day intervals starting when new growth appears in the spring may be used.
This article originally appeared in the May 25, 1994 issue, p. 79.