Monitoring of endophytic Brenneria salicis in willow and its relation to watermark disease

B. salicis was monitored in willow stands, sampling trees with and without watermark disease. The study focused on Salix alba and S. fragilis. B. salicis - presence was shown not to be restricted to diseased trees. With a B. salicis-specific PCR, B. salicis was frequently detected in the wood of healthy willows. This phenomenon was further studied with PCR-DGGE of endophytic bacterial populations in healthy and diseased willow. B. salicis concentrations were fluctuating during the year, synchronized with tree activity. Water stagnation in the wood as in winter was associated with a high B. salicis concentration, while the sap stream in active trees drained B. salicis from the wood vessels towards the leaves. We concluded that disease risk can not be assessed by testing the presence of B. salicis.