Brenneria salicis resides in symptomless willow (Salix spp.) and other tree species, but only willow trees develop watermark disease. To understand the conversion of B. salicis into a pathogen, its pathogenicity and differential growth in the various tree species are studied. Brenneria salicis was detected by plating and polymerase chain reaction-based techniques. Cell wall degradation and quorum sensing (QS) were assayed as possible pathogenicity mechanisms in wood. Differences in B. salicis growth capacities were tested in wood sap of the trees. Watermark diseased willow wood contained high concentrations of B. salicis with QS-induced cellulase activity. In the fall, wood sap of willow, and not of poplar and alder, promoted high density growth of B. salicis. In situ, B. salicis was the dominant bacterial type in willow wood during the fall and winter period. Willow sustains high densities of B. salicis at the time of leaf shedding. The cellulase in the immobilized wood sap has then a long-lasting contact with the xylem cell wall. Timing of dormancy and subsequent winter conditions might interfere with sap composition, B. salicis density, activity and survival, and be the reason, at least partly, for the variable occurrence of the disease.