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Ecology and Epidemiology

Occurrence and Properties of Copper-Tolerant Strains of Pseudomonas syringae Isolated from Fruit Trees in California. G. L. Andersen, Department of Plant Pathology, University of California, Berkeley 94720; O. Menkissoglou(2), and S. E. Lindow(3). (2)Department of Agriculture, Aristotelian University of Thessaloniki, Thessaloniki, Greece; (3)Department of Plant Pathology, University of California, Berkeley 94720. Phytopathology 81:648-656. Accepted for publication 6 February 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-648.

Approximately 40% of the ice-nucleation-active strains of Pseudomonas syringae isolated from asymptomatic leaves and flowers from almond and navel 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.