The lilac pathogen is listed as Pseudomonas syringae pv. This pathovar infects numerous hosts in addition to lilac. The pathovar has been further divided into isolates based on pathogenicity since no single isolate is capable of infecting all the hosts listed for this pathovar. Among the lilacs, varieties with white flowers are generally the most susceptible. The bacterium overwinters in infected bark and on twigs and bud scales.
Bacterial cells enter leaves and twigs through natural openings.
They may also enter young stems through infected leaves. Once the pathogen has entered the host it multiplies in intercellular spaces and produces a phytotoxin called syringomycin. The toxin destroys plant cell membranes and causes symptom expression. It is translocated through the xylem to young developing leaves. Infection is normally limited to current season growth.