Scientists are gaining a better understanding of Pierce’s disease and how it affects grapevines. The disease, which annually costs California more than $100 million, comes from a bacterium called Xylella fastidiosa. While the bacterium has been present in the state for more than 100 years, Pierce’s disease became a more serious threat to agriculture with the arrival of the glassy-winged sharpshooter insect, which can carry the bacterium from plant to plant.