Urban trees seem to be especially susceptible, perhaps because of the stresses they suffer: in American cities, some streets have had to be clear-felled. There is no known cure.
Cultivated trees have less natural resistance, being heavily treated with insecticide sprays and anti fungus and virus chemicals.
Xylella has ripped through olive groves in Italy and vineyards and fruit farms in the Americas.Philaenus spumarius, the froghopper, is a common vector
The disease has been called olive quick decline syndrome (OQDS; in Italian: complesso del disseccamento rapido dell’olivo). The disease causes withering and desiccation of terminal shoots, distributed randomly at first but which then expands to the rest of the canopy.
This results in the collapse and death of the trees. In the affected groves, all of the plants show symptoms. By the beginning of 2015 it had infected up to a million trees in the southern region of Apulia.By July 2015, Xylella fastidiosa had reached Corsica, by October 2015, it had reached Mainland France, near Nice, in Provence-Alpes-Côte d’Azur, affecting the myrtle-leaf milkwort (Polygala myrtifolia). This is the subspecies X. fastidiosa subsp. multiplex which is considered to be a new genetic variant of the bacterium, different to that found in Italy.
On 18 August 2016 in Corsica, 279 focuses of the infection have been detected, concentrated mostly in the south and the west of the island.
In August 2016, the bacterium has been detected in Germany in an oleander plant.
In late June 2017 the bacterium was found in Valencia, Spain see here.. http://www.levante-emv.com/economia/2017/06/30/bacteria-arrasa-almendros-olivares-baleares/1587567.html