Harvested grape clusters as inoculum for Pierce's disease.

Harvested fruit clusters from grapevines with Pierce's disease (PD) did not serve as sources from which an efficient insect vector (the blue-green sharpshooter, Graphocephala atropunctata) acquired the causal bacterium Xylella fastidiosa. Sharpshooters fed for 6 hr on fruit clusters harvested from PD-infected vines, then were tested twice for X. fastidiosa by exposure to healthy grapevines. Clusters were tested as possible sources 1, 7, 14, and 21 days (stored at 4 C) after being harvested from vines confirmed as having PD. None of 420 surviving bluegreen sharpshooters or 84 green sharpshooters (Draeculacephala minerva) from all tests transmitted the bacterium to grape, but 88% of 49 blue-green sharpshooters and 24% of 37 green sharpshooters surviving from these tests and then given a 6-hr access on diseased grapevines subsequently transmitted X. fastidiosa to grape. Isolations of X. fastidiosa from cluster stems and rachises were successful in only 5 of 24 samples 1 day after harvest. Concentrations of X. fastidiosa isolated from stems of diseased clusters were about 10-100 times lower than typical concentrations in grape petioles or leaf veins, decreased each week, and were not recovered after storage for 3 wk. Post-harvest fumigation with sulfur dioxide did not affect rates of recovery of X. fastidiosa from grape cluster stems