Eventually, infected plant parts turn completely black, then wither and die. 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. 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 bacterial blight.
Also, avoid wounding plants as bacteria enter through open wounds. There are chemical sprays available for bacterial blight but 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.