Overview of lilac bacterial blight
In early spring when the weather is cool and wet, the bacterium Pseudomonas syringae can infect newly emerging shoots, flower buds, and leaves on many lilac varieties, including Chinese, Japanese, Persian, and common lilac.
Symptoms of lilac bacterial blight
Initial symptoms include brown, water-soaked spots on leaves. Spots are initially pin-point in size but can enlarge to 1/8 inch or more. As the disease progresses, spots tend to coalesce, often causing leaves to become miss shapened. 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 new shoots develop in the spring. Shoots turn a black color, droop over, and die.
Disease cycle of lilac bacterial blight
The bacterium Pseudomonas syringae can overwinter in plant debris, healthy tissue, diseased cankers, perennial weeds, and soil. During wet weather in the spring, the bacterium spreads to new growth by wind, splashing rain, insect vectors, or on pruning tools. The bacterium requires a natural opening or wound to cause infection.
Type of Sample Needed for Diagnosis and Confirmation
The Iowa State University Plant & Insect Diagnostic Clinic can help you to investigate and confirm if you plant has this disease. Please see our website for current forms, fees, and instructions on collecting and packing samples. Contact information for each states diagnostic laboratory for U.S. residents. If your sample is from outside of Iowa please do not submit it to the Plant and Insect Diagnostic Clinic without contacting us
Management of lilac bacterial blight
Buy only disease-free plants. Good sanitation will help prevent the spread of bacteria to nearby healthy lilac plants. Immediately remove and destroy diseased plant parts. Remember to dip your pruners in a 10% bleach solution between each cut. Prune only when the weather is dry and no rain is expected within the next few days.
Sanitation combined with other cultural strategies can provide a good control of lilac bacterial blight. These include spacing or thinning plants to provide for good air circulation and proper watering to avoid wetting of foliage. Also, try to avoid wounding plants as bacteria enter through open wounds.
There are chemical sprays available for lilac bacterial blight; however, these sprays are often not feasible from a homeowner’s perspective. Most chemicals require a full coverage spray before disease appears in spring at an interval of 7-10 days and especially after each rain. Sprays can also cause phytotoxicity in some lilac varieties and does not guarantee complete disease control.