The legendary olive trees of Puglia produce some of the finest oil in the world. Thousands of farm families have pressed the fruits for generations. The trees' twisted trunks—some radiocarbon-dated at more than 2,500 years old—are as fundamental to the landscape here as the castles and the sea. They have persisted through centuries of invasions, wars, droughts and depressions. No matter how bad things have ever gotten, the orchards have always provided promise for the future. That's why the spontaneous death of these trees, presumably by a foreign bacterium called Xylella fastidiosa, feels like a black plague. Carried by insects, the bacterium has swept through grove after grove.