Variability of Virulence in Grapevine Among Isolates of the Pierce’s Disease Bacterium

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Abstract
The average virulence of Pierce''s disease (PD) bacterial isolates obtained from various grape species (Vitis vinifera, V. labrusca, V. rotundifolia, V. munsoniana, and other Vitis spp.) in V. vinifera ''Carignane'' and V. rotundifolia ''Carlos'' were similar. There was no indication of any host-specific race for grape species among these grape isolates. PD isolates from American elder (Sambucus canadensis), however, were less virulent in Carignane than were isolates from grape, and failed to produce symptoms in Carlos muscadine grape. Symptoms in the more resistant muscadine grapevine appeared to depended more on the level of virulence of the individual PD isolate rather than on its host of origin. PD isolates from all host species varied in virulence from avirulent to highly virulent, even if all the isolates had been obtained from a single plant. Virulence was lost during 18 mo. of serial transfers.