Physiological and Pathological Characteristics of Virulent and Avirulent Strains of the Bacterium that Causes Pierce's Disease of Grapevine

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Abstract
Virulence of strains of Pierce''s disease bacteria [rickettsia-like] to grapevine (Vitis vinifera ''Carignane'') was determined by the multiplication of bacteria in the xylem vessels of inoculated petioles or stem internodes and by their capability to move systemically from vessel to vessel through the plant. Virulent strains reached populations of 106-107 colony-forming units/cm of inoculated petiole and avirulent strains reached only 104-105 colony-forming units/cm. Virulent strains moved systemically from the inoculated internode and infested xylem vessels in all but the youngest grapevine tissue within 3 mo. Avirulent strains did not move beyond the inoculated internode. Weakly virulent strains multiplied more slowly in petioles than virulent ones but eventually reached 106 colony-forming units per centimeter, and systemic movement of weakly virulent strains was slower. None of the strains that cause Pierce''s disease produced pectolytic, cellulolytic or proteolytic enzymes in culture. Peroxidase and lysophospholipase were produced: virulent strains produced the most lysophospholipase.