Bacterial pathogens thrive in wet conditions, especially in mild spring temperatures when plenty of succulent (susceptible) new growth is present. For these reasons, bacterial blight of lilac was expected this wet 2008. The causal bacterium is a Pseudomonas species that is common in the Midwest.
Symptoms of bacterial blight vary with the weather conditions and the host. Brown or black spots on the foliage may melt together to cause large necrotic areas. Often a yellow halo is present around the blotches. Young shoots may be partially or entirely brown. When young leaves are infected, they die quickly and resemble fire blight. The image shows a recent Plant Clinic sample of lilac infected with bacterial blight.
Laboratory confirmation involves observing bacterial exudate from the affected tissues and isolating the causal bacterium.
Disease management requires removal of infected tissue, cutting several inches into the good wood, below any stem cankers. As with fire blight management, prune only in dry weather and be certain to disinfect clippers between cuts with a disinfectant such as 10% Clorox. Rapid drying of plant tissue helps discourage disease, so try to open the area via pruning to allow better air movement and thus quicker drying. If the plants are growing in an area of overhead irrigation, cease that activity, at least in the spring. Instead, water the soil directly. If you need to fertilize, use only balanced fertilizer. High nitrogen levels promote succulent new growth that is more susceptible to infection. Finally, copper sulfate fungicides provide some control of the disease but must be applied before new growth appears and be repeated to keep new growth protected through spring.
Bacterial blight has been found to be more of a problem on stressed plants, so practice sound horticultural methods to promote healthy plants. Some researchers have reported the disease is worse on white flowering varieties.