The Apocalypse of Salento. Xylella fastidiosa is an aerobic, Gram-negative bacterium of the monotypic genus Xylella. It is a plant pathogen, and is transmitted exclusively by xylem fluid feeding sap insects. Many plant diseases are due to symptomatic 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) and citrus variegated chlorosis (CVC). In Europe it has attacked olive trees in the Salento area of Southern Italy causing the olive quick decline syndrome. While distributions of X. fastidiosa–related diseases are mostly limited to the Americas, outbreaks have occurred in Taiwan, Slovakia, and other countries worldwide.
Many plants are asymptomatic carriers of the bacteria, which can contribute to its spread. Pathogenicity of the bacterium occurs only when a large proportion of xylem vessels in a plant are colonized; often, the colonies of bacteria themselves are not large enough to completely block the vessels, and the mechanism of pathogenesis is largely unknown. A subspecies of X. fastidiosa responsible for citrus variegated chlorosis was the first non-viral plant pathogen to have its genome sequenced, in part because of its potential to devastate affected crops.