VIEW ARTICLE

Disease Control and Pest Management

Toxicity of Fungicidal Copper in Soil to Citrus Seedlings and Vesicular-Arbuscular Mycorrhizal Fungi. J. H. Graham, Assistant professor of soil microbiology, University of Florida, IFAS, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred 33850; L. W. Timmer(2), and D. Fardelmann(3). (2)(3)Professor of plant pathology, and research assistant in soil science, respectively, University of Florida, IFAS, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred 33850. Phytopathology 76:66-70. Accepted for publication 30 July 1985. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/Phyto-76-66.

Carrizo citrange (Poncirus trifoliata x Citrus sinensis) seedlings were potted in a sand soil (pH 6.8) amended with eight rates of copper (0-300 μg·g^-1 of soil) as basic copper sulfate (CuSO₄·3 Cu(OH)₂·H₂O). Double-acid-extractable Cu concentrations in soil ranged from 3 to 248 μg·g^-1.Growth of seedlings and colonization by the mycorrhizal fungus Glomus intraradices were reduced logarithmically with Cu concentration. Minimum toxic amounts of Cu ranged from 19 to 34 μg·g^-1 of soil. Leaf P content decreased linearly with Cu for mycorrhizal seedlings without added P but not for nonmycorrhizal plants with supplemental P. The Cu-induced reduction in P uptake of mycorrhizal plants was more closely related to the inhibition of hyphal development outside of the root than to development of vesicles and arbuscules in the root. Thus, Cu-induced P deficiency was attributed to inhibition of P uptake by mycorrhizal hyphae in soil. In a citrus orchard soil with double-acid-extractable Cu > 80μg·g^-1 and pH < 5, replanted trees were stunted and had less mycorrhizal colonization than unaffected trees.