The first withered olive trees appeared near Gallipoli, in the Apulia region of southern Italy. Bunches of leaves turned brown and crunchy around the edges. Then, whole groves started to wane. Farmers whose families had tended olives for generations watched their trees dry up and their businesses plummet.

At first, it wasn’t clear what was causing the decline. Was it a fungus? A virus? Something else entirely? Scientists showed up in the olive groves to sample the trees, urgently trying to find the cause.

One researcher from a local agricultural institute had just come back from a conference in California, where he’d learned about the plant bacterium Xylella fastidiosa. The symptoms the olive growers were seeing, he realized, looked exactly like