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Increased Toxicity of Iron-Amended Copper-Containing Bactericides to the Walnut Blight Pathogen Xanthomonas campestris pv. juglandis. Y. -A. Lee, Department of Plant Pathology, University of California, Berkeley 94720, Present address: Department of Biology, Fu Jen University, HsinChuang, Taipei, Taiwan, Republic of China; M. N. Schroth(2), M. Hendson(3), S. E. Lindow(4), X.-L. Wang(5), B. Olson(6), R. P. Buchner(7), and B. Teviotdale(8). (2)(3)(4)(5)Department of Plant Pathology, University of California, Berkeley 94720; (6)(7)Cooperative Extension, University of California, Butte County and Tehama County; (8)Kearney Agriculture Center, Parlier, CA 93648. Phytopathology 83:1460-1465. Accepted for publication 13 September 1993. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/Phyto-83-1460.

The addition of iron to fixed copper compounds enhanced the toxicity of these materials to the walnut blight bacterium Xanthomonas campestris pv. juglandis by several mechanisms. Iron altered the physiology of the bacterium, causing it to be more sensitive to cupric ions. In the presence of iron, less free copper ions were needed to inhibit the growth of bacteria. The addition of soluble iron salts to fixed copper compounds also reduced pH and increased the amount of free copper ions; addition of iron caused the release of more free copper ions in suspensions of Kocide 101 (77% Cu(OH)2) than did hydrochloric acid at the same pH. The spraying of walnut leaves with Kocide 101 amended with iron salts significantly increased the availability of free copper ions on leaf surfaces compared to the bactericide alone. Cells of copper-sensitive strains of X. c. juglandis did not survive on leaf surfaces treated with Kocide 101, whereas more than 25% of the cells of copper-resistant strains survived. The addition of iron (50 g/ml) in the form of FeCl36H2O to Kocide 101 increased the concentrations of free copper ions by more than 25-fold and eliminated copper-resistant strains on leaf surfaces. In field trials, combinations of Kocide 101 or Champion (77% CU(OH)2) with iron significantly reduced the incidence of bud infestation and blighted leaflets compared to non-amended treatments.