The Xanthomonadaceae are a family of Gram negative bacteria belonging to the order Xanthomonadales in the gammaproteobacteria. They are typically characterized as environmental organisms and are found in soil and water, as well as plant tissues. Many Xanthomonadaceae, especially species from the genera Xanthomonas and Xylella, cause plant diseases. Only one, Stenotrophomonas maltophilia, has isolates known to be opportunistic human pathogens.

Xylella fastidiosa is a gammaproteobacteria that causes lethal disease in a variety of different plants. Spread of disease from plant to plant seems to obligatorily require an insect vector (leafhoppers and sharpshooters). Vines, almond trees, citrus, oleander, peach, pear, coffee, maple, mulberry, elm, sycamore and alfalfa can all be infected by this bacterium. The bacteria infect the xylem where it forms a gel, preventing water movement within the plant. Large-scale infections are economically devastating. Strain M23 was isolated from almond trees in the San Joaquin Valley (California), USA in 2003. It causes both almond leaf scorch disease and Pierces disease, and is of genotype G.