EFFECT OF COPPER NUTRITIONAL SPRAY CHEMISTRY ON APPLE FRUIT RUSSETTING AND LEAF TISSUE COPPER CONCENTRATION

Copper (Cu) concentration in apple trees has been declining in commercial orchards, probably because of reduced use of Cu-based pesticides and greater purity of fertilizer materials. As a consequence, there is increasing use of Cu-containing foliar sprays applied expressly for nutritional purposes. The current study evaluated the relationship of Cu geochemistry in Cu nutritional sprays to apple fruit russetting and leaf tissue Cu status. The Cu source was a proprietary nutrient spray product formulated from basic copper sulfate and dicalcium phosphate. The field experiment was conducted in an irrigated 'Golden Delicious’ apple orchard located in semiarid Wenatchee, Washington, USA. Treatments included two mid-summer foliar sprays of: (1) water; (2) Cu sprays containing differential concentrations of Cu²⁺(aq) free metal ion produced by acidifying a fixed concentration of the Cu product with H₃PO₄ (P series); and (3) Cu sprays containing differential concentrations of CuCl ion-pairs made by reacting CaCl₂ with the Cu source (Cl series). The thermochemical equilibrium-based computer program MINTEQA2 was used to chemically speciate the components of laboratory and field spray suspensions. Measured total dissolved Cu concentrations in the suspensions corresponded well with MINTEQA2 predictions. The experimental treatments had no effect on apple fruit russetting. Leaf Cu concentration was strongly related to total Cu system concentration (aqueous plus solid phases) in the spray suspensions. In contrast, leaf Cu concentration was poorly related to total dissolved Cu or the modeled concentrations of Cu²⁺(aq) or any of the Cu-P, Cu-Cl, Cu-OH, Cu-S dissolved ion-pair species. The experimental results indicate that russetting and leaf Cu concentrations are poorly related to Cu geochemistry in Cu nutrient spray suspensions as initially prepared. Copper spray residues adhering to leaf surfaces may over-estimate Cu absorption, and/or Cu phytoavailability from foliar sprays may be better modeled by incorporating into the geochemical model the effects of evapoconcentration on solution composition and solid phase formation.