Seasonal Concentration of the Pierce’s Disease Bacterium in ‘Carlos’ and ‘Welder’ Muscadine Grapes Compared with ‘Schuyler’ Bunch Grape

The accumulation of Pierce’s disease bacterial occlusions in xylem vessels of leaves of susceptible ‘Schuyler’ bunch grapes occurred 2 months earlier than the accumulation in leaf vessels of the more tolerant ‘Carlos’ and ‘Welder’ muscadines. The first appearance of leaf marginal necrosis symptoms occurred when the accumulation of bacterial occlusions was approaching the maximum—the last week of June in the bunch grape, the last week of Aug. in the 2 muscadines. The site of bacterial accumulation was primarily in the leaves of ‘Carlos’, but in both leaves and stems of ‘Schuyler’ and ‘Welder’. Stem dieback symptoms are more common in the latter 2 cultivars.