Xylella fastidiosa is the causal agent of oleander leaf scorch disease
A lethal leaf scorch disease of oleander appeared in Southern California (US) in the 1990s. It was fist found in the Palm Springs area (Riverside county). It then spread to other Californian counties (Orange, San Diego, San Bernardino) and also to Texas. Xylella fastidiosa (EPPO A1 quarantine pest) was strongly suspected to be the cause of this disease (see EPPO RS 97/049). Further studies have now confirmed this hypothesis. X. fastidiosa was detected by isolation on growing medium, ELISA and PCR in most symptomatic plants (but not in asymptomatic plants or negative controls). Mechanical inoculations of oleanders with bacterial cultures (obtained from oleanders) produced symptoms of oleander leaf scorch, and X. fastidiosa could then be re-isolated from these inoculated plants (thus verifying Koch's postulates). Three leafhoppers species feeding on xylem sap (Graphocephala atropunctata, Homolodisca coagulata and H. lacerta) were able to transmit the disease from oleander to oleander. After mechanical inoculation, no bacterium could be re-isolated from grapevine (Vitis vinifera), peach (Prunus persica), olive (Olea europea), Rubus ursinus and valley oak (Quercus lobata). Sequence comparison (500 bp sequence of 16S-23S rRNA spacer region) of oleander strains showed 99.2 % identity with Pierce's disease strains, 98.4% with oak leaf scorch strains, 98.6% with peach phony, plum leaf scald and almond leaf scorch strains.
Sources
Purcell, A.H.; Saunders, S.R.; Hendson, M.; Grebus, M.E.; Henry, M.J. (1999) Causal role of Xylella fastidiosa in oleander leaf scorch disease.
Phytopathology, 89(1), 53-57.