Leaves infected with the bacterial blight organism (Pseudomonas syringae) show brown spots that are surrounded by a yellow halo. Spots are initially pinpoint in size but can enlarge to 1/8 inch or more. As the disease progresses, spots tend to coalesce, causing leaves to become misshapen. 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 shoots develop in the spring. Diseased shoots turn black, droop over, and die.
Control of bacterial blight consists of pruning out blighted twigs as soon as they occur. Cut several inches below the margin between healthy and diseased tissue. Prune in dry weather. Pruners should be dipped in a 10% bleach solution or 70% alcohol between cuts to prevent spreading the disease. Improving air circulation by properly thinning plants also will help prevent disease problems.
Copper based fungicides such as Bordeaux mixture or Phyton 27 can be applied when the disease first appears. Follow label directions.
** Note ** Cold injury in lilacs is sometimes mistaken for an infectious disease. Near-freezing temperatures in the spring can cause browning or blackening of leaf tissue, especially at the margin. Leaves eventually appear torn around the edges and along the veins. Puckering of leaves also may occur.
This article originally appeared in the May 23, 1997 issue, p. 76.