AILSA CHANG, HOST:
Scientists in southern Europe are fighting a deadly pathogen that's killed millions of olive trees in Italy. And trees in Spain and Greece are under threat. These countries produce nearly all of Europe's olive oil. And as Joanna Kakissis reports, the disease has no known cure.
JOANNA KAKISSIS, BYLINE: Federico Manni first noticed something was wrong with his family's olive trees more than six years ago. It was summer. The cicadas were singing, and he and his dad were checking their olives in southern Italy. They noticed some trees looked burnt.
FEDERICO MANNI: Dead branch, brown leaves - it's terrible. It's really, really terrible.
KAKISSIS: Most of their trees eventually died. They now call the field worked by generations of their family an olive tree cemetery.
MANNI: My father, he remembers his father, he remembers his grandfather worked this field. It's not a field. It's like a house. When we lost the trees, we lost our house.
KAKISSIS: The tree killer is a bacterium called xylella fastidiosa. And it's originally from the Americas. Plant virologist Maria Saponari says xylella probably arrived in Italy with ornamental coffee plants.
MARIA SAPONARI: This pathogen likes a warm environment. And so unfortunately, it found a very suitable condition to establish here.
KAKISSIS: Saponari compares xylella to the coronavirus. Just as COVID-19 keeps oxygen from reaching your vital organs, xylella clogs olive trees so they can't absorb water.
SAPONARI: And so the tree dies.
KAKISSIS: The pathogen is sapped (ph) out by insects like the spittlebug and spread when the insect feeds on another plant.
SAPONARI: We cannot cure the infected plants. So prevention is very important to reduce the expansion of the infection.
KAKISSIS: In Southern Italy's Puglia region where olive trees define the landscape, farmers clean infected branches with copper sulfate. They quarantine sick trees and uproot healthy ones nearby. But Gianni Cantele, who leads the regional growers association, says the disease has cut the supply of olive oil.
GIANNI CANTELE: The production in this area has brought in no more than 10% of the original.
KAKISSIS: A new study shows that xylella could cost Southern Europe more than $20 billion. In Spain, it has mainly affected almond trees and vineyards. But plant pathologist Blanca Landa worries that lax controls on imported plants...
BLANCA LANDA: Can introduce something that can be really dangerous and destroy completely the economy of a country.
KAKISSIS: Xylella has not yet infected olive trees in Greece, but farmers are on guard. Antonis Marakakis, the lead agronomist at an olive cooperative on the island of Crete, tells farmers to call him immediately if trees look sick.
ANTONIS MARAKAKIS: (Through interpreter) I tell them to get samples from the tree so it can be tested right away for signs of xylella. We have to catch it before it spreads.
KAKISSIS: Back in southern Italy, Federico Manni's family is now farming potatoes. He sends me old videos of his family harvesting and pressing olives before xylella turned their trees into scorched grey corpses.
MANNI: It's too late for our trees. We can only replace the trees. If we plant 20% of what we lost, it's a good number.
KAKISSIS: But he says this time he will plant different olive varieties, ones that scientists hope may prove resistant to xylella.
For NPR News, I'm Joanna Kakissis. Transcript provided by NPR, Copyright NPR.