Relative Sensitivity of Selected Grapevine Cultivars to Pierce's Disease Bacterial Inoculations

Dormant cuttings of each of 25 grape cultivars were inoculated with 0.1 mL of Pierce9s disease (PD) bacterial suspension (108 cells/mL) in distilled water. Cuttings of each cultivar inoculated with sterile distilled water were used as controls. All the cuttings were rooted and kept in an insect-proof greenhouse. Each of the inoculated cultivars showed marginal leaf burning after three months, whereas uninoculated controls remained symptomless. The PD symptoms varied among grape cultivars. French Colombard and Chardonnay showed severe leaf burning symptoms, whereas Chenin blanc and Sylvaner exhibited little leaf burning. The concentration of PD bacterium in each grape cultivar was evaluated by enzymelinked immunosorbent assay (ELISA). In general the absorbance values (A405 nm) in ELISA were correlated with intensity of symptom expression of different cultivars. Chenin blanc and Sylvaner were the most tolerant, whereas French Colombard and Sauvignon blanc were the least. Grape sap has a relatively small effect on the A405 nm values and the absorbance values in ELISA seem to be directly proportional to bacterial concentration. Intensity of disease development in commercial vineyards appear to relate with the mean A405 nm values of Chenin blanc, Flora and Pinot noir.