POPULATIONS OF XYLELLA-FASTIDIOSA IN PLANTS REQUIRED FOR TRANSMISSIONBY AN EFFICIENT VECTOR

Xylella fastidiosa, a xylem-limited bacterium that causes Pierce's dis ease (PD) of grapevine and other diseases, is transmitted efficiently by xylem-feeding leafhoppers. Acquisition of a PD strain of X fastidio sa by the blue-green sharpshooter (BGSS) from five plant host species- grapevine (Vitis vinifera), Himalayan blackberry (Rubus discolor), Cal ifornia mugwort (Artemisia douglasiana), watergrass (Echinochloa crus- galli), and Bermuda grass (Cynodon dactylon)-was tested at various tim e intervals after vector inoculation. The minimum incubation periods i n plant hosts before BGSS acquired X. fastidiosa were 4, 22, 29, and 2 5 days for grapevine, blackberry, mugwort, and watergrass, respectivel y. There were no transmissions by vectors or recoveries of X. fastidio sa by culturing from Bermuda grass in 133 attempts, including 80 attem pts with the green sharpshooter, Draeculacephala minerva. The first ac quisitions and subsequent transmissions by BGSS occurred after X. fast idiosa multiplied to a population of about 10(4) CFU/g of stem tissue. Higher populations of bacteria in plants resulted in higher rates of transmission. In grapevine, the rate of transmission increased over ti me (4.5% in the first 10 days to 55% after day 25) as the maximum numb er of viable CFU of X. fastidiosa recovered by culturing also increase d (from 5 x 10(5) CFU/g during the first 10 days to 5 x 10(8) after da y 25).