OCCURRENCE AND PROPERTIES OF COPPER-TOLERANT STRAINS OF PSEUDOMONAS-SYRINGAE ISOLATED FROM FRUIT-TREES IN CALIFORNIA

Approximately 40% of the ice-nucleation-active strains of Pseudomonas syringae isolated from asymptomatic leaves and flowers from almond and naval orange trees from orchards with a history of copper usage were able to grow in a culture medium amended with 0.32 mM CuSO4. While more than one-half of the strains were highly sensitive to cupric ions, some strains could tolerate as much as 1.12 mM CuSO4 in a culture medium. Prior exposure of copper-tolerant strains to sublethal concentrations of copper in a culture medium increased the fraction of cells that could survive a higher concentration of copper by more than 1,000-fold compared with cells not receiving copper pretreatment. The mean LC50 of copper-tolerant strains in aqueous copper solutions (23 ppb Cu+2) was about five times that of copper-sensitive strains (4.7 ppb) when cells were assayed without prior exposure to Cu+2 in growth medium. The LC50 of copper-tolerant strains increased to approximately 160 ppb when cells were grown in medium containing sublethal concentrations of CuSO4. Copper-tolerant strains of P. syringae grew as rapidly on Cu(OH)2-treated leaves as on nontreated leaves, while sensitive strains showed little growth. The size of established epiphytic populations of copper-sensitive but not copper-tolerant strains was reduced significantly on treatment of bean and almond leaves with Cu(OH)2 under greenhouse and field conditions.