Toxicity of Fungicidal Copper in Soil to Citrus Seedlings and Vesicular-Arbuscular Mycorrhizal Fungi

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Abstract
Carrizo citrange (Poncirus trifoliata .times. Citrus sinensis) seedlings were potted in a sand soil (pH 6.8) amended with eight rates of copper (0-300 .mu.g .cntdot. g-1 of soil) as basic copper sulfate (CuSO4 .cntdot. 3 Cu(OH)2 .cntdot. H2O). Double-acid extractable Cu concentrations in soil ranged from 3 to 248 .mu.g .cntdot. 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 .mu.g .cntdot. 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 < 80 .mu.g .cntdot. g-1 and pH < 5, replanted trees were stunted and had less mycorrhizal colonization than unaffected trees.