HARVESTED GRAPE CLUSTERS AS INOCULUM FOR PIERCES DISEASE

Harvested fruit clusters from grapevines with Pierce's disease (PD) di d not serve as sources from which an efficient insect vector (the blue -green sharpshooter, Graphocephala atropunctata) acquired the causal b acterium Xylella fastidiosa. Sharpshooters fed for 6 hr on fruit clust ers harvested from PD-infected vines, then were tested twice for X. fa stidiosa by exposure to healthy grapevines. Clusters were tested as po ssible sources 1, 7, 14, and 21 days (stored at 4 C) after being harve sted from vines confirmed as having PD. None of 420 surviving blue-gre en sharpshooters or 84 green sharpshooters (Draeculacephala minerva) f rom all tests transmitted the bacterium to grape, but 88% of 49 blue-g reen sharpshooters and 24% of 37 green sharpshooters surviving from th ese tests and then given a 6-hr access on diseased grapevines subseque ntly transmitted X. fastidiosa to grape. Isolations of X. fastidiosa f rom cluster stems and rachises were successful in only 5 of 24 samples 1 day after harvest. Concentrations of X. fastidiosa isolated from st ems of diseased clusters were about 10-100 times lower than typical co ncentrations in grape petioles or leaf veins, decreased each week, and were not recovered after storage for 3 wk. Post-harvest fumigation wi th sulfur dioxide did not affect rates of recovery of X. fastidiosa fr om grape cluster stems.