Harvested grape bunches are not sources of Xylella fastidiosa inoculum
Studies have been carried out in California (US) in order to determine whether grape bunches could serve as feeding aquisition sources for efficient insect vectors of Xylella fastidiosa (EPPO A1 quarantine pest), which could then transmit the disease to other plants. The most efficient vectors are Graphocephala atropunctata and to a lesser extent Draeculacephala minerva. Insect vectors fed for 6 h on bunches harvested from infected grapevines, and were tested twice for X. fastidiosa by exposure to healthy grapes. Bunches were tested as possible sources 1, 7, 14 and 21 days after harvest (from infected grapevines). None of the 420 surviving Graphocephala atropunctata or 84 Draeculacephala minerva transmitted the bacterium to grape. However, when using the same insects but feeding on infected grape foliage instead of bunches, high rates of transmission were obtained. Isolation of the bacteria from stems and rachises of bunches was successful in only 5 to 24 samples, one day after harvest. Concentrations of X. fastidiosa isolated from stems of diseased bunches were about 10 to 100 times lower than typical concentrations in grape petioles or leaf veins. These concentrations decreased each week, and bacteria were not recovered after storage for 3 weeks. The authors felt that this information would be useful in assessing quarantine measures to prevent the introduction of X. fastidiosa in shipments of fresh grapes.
Sources
Purcell, A.H.; Saunders, S. (1995) Harvested grape clusters as inoculum for Pierce's disease.
Plant Disease, 79(2), 190-192.