Bacterial blight of lilac is caused by the bacterium Pseudomonas syringae. Infection results in brown spots on leaves and internodes. These spots may enlarge and coalesce, causing leaves to become misshaped. 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 new shoots develop in the spring. The 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 bleach or 70% alcohol between cuts to prevent spreading the disease. Cut several inches below the margin between healthy and diseased tissue. Prune in dry weather only. As a preventative measure, thin plants to provide good air circulation. You may spray a copper- based product, such as Bordeaux mixture, 2 to 3 times at intervals of 7 to 10 days, starting when new growth appears in the spring.
This article originally appeared in the May 19, 1995 issue, p. 72.