Across Europe, crops and ornamental plants are being hammered by a destructive bacterial invader called Xylella fastidiosa. Now a genomic analysis shows that one of the most threatening forms of this bacterium probably invaded Europe multiple times, in most instances arriving from California.

Since 2013, X. fastidiosa subspecies multiplex has been found in almond trees (Prunus dulcis) in Spain and fig trees (Ficus carica) in Italy. It infects more of Europe’s plant species and occurs across a wider swathe of the continent than any other subspecies, including the subspecies pauca, which is killing Italy’s iconic olive (Olea europaea) groves.

Blanca Landa at the Institute for Sustainable Agriculture in Cordóba, Spain, and her colleagues analysed the microbe’s entire genome and found that before spreading to Europe, the bacterium was probably introduced to California from the southeastern United States. The subspecies shows a high level of diversity in the southeast, suggesting that it originated there.

Researchers think that commercial trade of plant material drove the bacterium’s movements. Understanding the routes the microbe travelled could help European officials to prevent future outbreaks.