When you think of the Napa and Sonoma Valleys, you probably imagine rolling hills adorned with row upon row of succulent grapes vine-ripening in the sun. However, beneath this idyllic exterior the bacterium Xylella fastidiosa is wreaking havoc on California’s wine grapes. By cutting off water transport through the plants, Xylella causes leaves to wither and fall off, ultimately killing entire vines. In an effort to better understand the mechanism of this disease, Associate Specialist Clelia Baccari and Professor Steven Lindow of the Department of Plant and Microbial Biology are studying the movement of the bacteria through the tissues of resistant and susceptible grape varieties. They use a strain that expresses green fluorescent protein, making the bacterial cells easy to visualize under a microscope. The plant’s ray cells appear red, and between them are xylem vessels—water-transporting vascular tissue. The bright yellow-green areas on the walls of the xylem are full of bacteria. Dr. Baccari found that susceptible varieties, such as Cabernet Sauvignon on the left, had about five times as many infected xylem vessels as the Tampa grape, on the right, which is relatively resistant to the disease. Current hypotheses as to why some grape varieties are more resistant include differences in sap composition and the production of tyloses — outgrowths of cells surrounding xylem vessels, which may block bacterial movement through the plant.