Brenneria salicis

Brenneria salicis is a Gram-negative bacterium that is pathogenic on plants. The bacterium is known to cause 'watermark disease' in willow (Salix ssp.) trees. Watermark disease affects infected trees by occluding the xylem vessels, impeding circulation. Early signs of the disease are wilting or dried-out, dead leaves on intermittent branches throughout the tree's crown. Cross sections from affected branches show a watery, transparent discoloration to an orange-brown staining in the wood. Cross sections of a killed tree's main stem are blanched completely white. The white willow, S. alba, is very sensitive to watermark disease, while the crack willow, S. fragilis, is considerably less so. Hybrids of these two species have shown intermediate sensitivity.

from Wikipedia