How to save the world’s orange groves

CITRUS-GREENING disease is a bacterial infection of citrus-fruit trees, spread by insects called psyllids. It was first recorded a century ago, in China, and it has since spread widely. It can be extremely harmful. Within a decade of its arrival in Florida, for example, it had wreaked $4.6bn-worth of damage and reduced yields by 74%. If its spread continues, says Georgios Vidalakis of the University of California, Riverside, who directs California’s Citrus Clonal Protection Programme, citrus fruits risk becoming niche products.The problem is less the bacteria than the host plant’s reaction to them. They are injected when the insects feed on sap-carrying phloem tissues—the parts of a plant’s internal plumbing responsible for transporting sugar around. To stop the bacteria spreading, the plant mounts an immune response which thickens the walls of phloem cells with callose, a polymer made of sugar molecules. In the case of citrus-greening disease this response is overenthusiastic, and the phloem tubes get blocked.