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Disease Control and Pest Management

Chemical Forms of Copper on Leaves in Relation to the Bactericidal Activity of Cupric Hydroxide Deposits on Plants. O. Menkissoglu, Department of Agriculture, Aristotelian University of Thessaloniki, Thessaloniki, Greece; S. E. Lindow, Department of Plant Pathology, University of California, Berkeley 94720. Phytopathology 81:1263-1270. Accepted for publication 23 May 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-1263.

The total amount of copper, the amount of soluble but complexed copper, and the concentration of free Cu2+ ions on the surface of navel orange and bean leaves treated with different amounts of Cu(OH)2 or Bordeaux mixture were determined under field conditions. Total copper deposits decreased with time after spray application with apparent first-order kinetics with a half-life of approximately 45 and 35 days on navel orange and bean leaves, respectively. The concentration of soluble but complexed copper, however, increased for 2030 days following spray application and comprised approximately 25 and 10% of the total copper on treated navel orange and bean leaves, respectively. The concentration of free Cu2+ ions on Cu(OH)2-treated leaves increased with time after spray application to a maximum concentration of approximately 100 ppb after about 1020 days following treatment. No cells of copper-sensitive strains of Pseudomonas syringae survived after application to bean or navel orange leaves containing more than about 50 ppb of Cu2+ In contrast, 10% or more of the cells of copper-tolerant strains survived on leaves containing up to 100 ppb of free Cu2+, and nearly all of the cells of tolerant strains survived on leaves containing lesser amounts of free Cu2+. Strains of P. syringae survived only at Cu2+ concentrations on leaves at or below the LC50 for Cu2+ for these strains determined in distilled water in vitro. Thus, even though very low concentrations of Cu2+ are present on leaves, copper-sensitive strains are killed by the concentrations of free Cu2+ present on leaves. However, the concentration of free Cu2+ on leaves is only slightly less than that tolerated by copper-tolerant strains of P. syringae in vitro. Soluble but complexed forms of copper, while abundant on leaves, have no significant toxicity towards strains of P. syringae.