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. 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.