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Brenneria salicisresides in symptomless willow (Salixspp.) and other tree species, but only willow trees develop watermark disease. To understand the conversion ofB. salicisinto a pathogen, its pathogenicity and differential growth in the various tree species are studied.Brenneria saliciswas 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 inB. salicisgrowth capacities were tested in wood sap of the trees. Watermark diseased willow wood contained high concentrations ofB. saliciswith QS-induced cellulase activity. In the fall, wood sap of willow, and not of poplar and alder, promoted high density growth ofB. salicis. In situ, B. saliciswas the dominant bacterial type in willow wood during the fall and winter period. Willow sustains high densities ofB. salicisat 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. salicisdensity, activity and survival, and be the reason, at least partly, for the variable occurrence of the disease.