Xylella fastidiosa is an aerobic, Gram-negative bacterium of the genus Xylella. It is a plant pathogen, that grows in the water transport tissues of plants (xylem vessels) and is transmitted exclusively by xylem sap-feeding insects such as sharpshooters and spittlebugs. Many plant diseases are due to infections of X. fastidiosa, including bacterial leaf scorch, oleander leaf scorch, coffee leaf scorch (CLS), alfalfa dwarf, phony peach disease, and the economically important Pierce's disease of grapes (PD), olive quick decline syndrome (OQDS), and citrus variegated chlorosis (CVC). While the largest outbreaks of X. fastidiosa–related diseases have occurred in the Americas and Europe, this pathogen has also been found in Taiwan, Israel, and a few other countries worldwide.

Xylella fastidiosa can infect an extremely wide range of plants, many of which do not show any symptoms of disease. Disease occurs in plant species that are susceptible due to blockage of water flow in the xylem vessels caused by several factors: bacterial obstruction, overreaction of the plant immune response (tylose formation), and formation of air embolisms. A strain of X. fastidiosa responsible for citrus variegated chlorosis was the first bacterial plant pathogen to have its genome sequenced, in part because of its importance in agriculture. Due to the significant impacts of this pathogen on agricultural crops around the world there is substantial investment in scientific research related to X. fastidiosa and the diseases it causes.