Lilac Bacterial Blight

Need to know

Symptoms include brown, water-soaked spots that tends to coalesce as disease progresses.
The disease spreads during wet weather in the spring by wind, splashing rain, insect vectors, or on pruning tools.
Preventative measures include buying disease-free plants and proper sanitation practices.
Chemical sprays are available but are not feasible from a homeowner’s perspective.

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.

Image of Lilac Bacterial Blight — Lilac bacterial blight symptoms

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.

Signs of lilac bacterial blight

Bacterial ooze (streaming) may be evident when observed using a microscope. Culturing and other testing are recommended to confirm this problem as symptoms may resemble other diseases.

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. Select varieties that have tolerance or resistance to this pathogen. 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 good control of lilac bacterial blight. These include spacing or thinning plants to provide for good air circulation and proper watering to avoid the 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 the 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 do not guarantee complete disease control.

Fungicide applications may be avoided by following good Integrated Pest Management practices like those listed in this encyclopedia article. Often, the only preventative application is effective to manage plant diseases. If the problem requires a fungicide, state law requires the user to read and follow all labels accordingly. For more information, read Proper fungicide use.