One day in 2005, just before Hurricane Katrina blew through Florida and devastated New Orleans, Susan Halbert stood before a pomelo tree on a farm outside Miami. Something about this tree did not look right. It seemed undernourished: its leaves were sparse, and its melon-size citrus fruit was lopsided. Yet all the other plants in the garden were thriving, and the woman who took care of them had carefully tended the pomelo with a fresh layer of fertilizer. “She clearly knew how to grow plants,” says Halbert, an entomologist for the Florida Department of Agriculture and Consumer Services (FDACS).
Halbert scrutinized the tree like a detective at a crime scene, mentally ticking off every condition she could think of. She ruled out root rot, which is caused by a fungus, because the tree showed none of the characteristic signs of decay. Next, she considered a viral disease known as citrus tristeza—Spanish and Portuguese for “sadness”—which affects trees that have been grafted. (Citrus growers often raise trees not from seeds but by inserting a branch from one tree into the bark of another.) The pomelo, however, had not been grafted. Eventually Halbert got to the bottom of her list to the most devastating disease of citrus plants in the world—huanglongbing, Chinese for “yellow dragon disease.”