Brenneria salicis is a Gram-negative

Gram-negative

Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...

 bacterium that is pathogenic on plant

Plant

Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

s.

The bacterium is known to cause 'watermark disease' in willow (Salix ssp.) trees. Watermark disease affects infected trees by occluding the xylem

Xylem

Xylem is one of the two types of transport tissue in vascular plants. . The word xylem is derived from the Classical Greek word ξυλον , meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant...

 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.