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Comparison of Dormant and Circum-bloom Zinc Spray Programs for Washington Apple Orchards

ABSTRACT Spraying high rates of inorganic zinc (Zn) salts during the late-dormant or delayed-dormant period has been a traditional practice to control Zn insufficiency in Pacific Northwest apple (Malus × domestica Borkh.) orchards. Long-term use of such sprays can cause excessive soil accumulation of Zn. Using circum-bloom sprays of less phytotoxic Zn products may improve Zn phytoavailability and reduce Zn fertilizer requirements. A four-year field study was conducted comparing Zn phytoavailability and other effects of five Zn spray programs applied to ‘Golden Delicious’ apple trees. Treatments were late-dormant sprays of zinc sulfate (ZnSO4) applied at medium and high rates, a single delayed-dormant spray of a medium rate of zinc oxide (ZnO) tank-mixed with horticultural oil, and multiple circum-bloom sprays of two organically complexed Zn products applied at much lower per-acre rates. No sprays were applied in the last year in order to evaluate carry-over effects. Zinc concentrations of unwashed and detergent plus acid-washed leaves (a procedure used to remove surface residues of the Zn sprays) sampled in August and of unwashed winter buds were used as indices of tree Zn status. The late- and delayed-dormant Zn sprays had no effect on leaf Zn concentration, which also was unaffected by washing in these treatments. The organically complexed Zn sprays substantially increased Zn concentrations in both unwashed and washed leaves. Washing reduced leaf Zn concentration in these treatments. There was no detectable lasting effect of the three previous seasons of Zn sprays on leaf Zn in the fourth year, or on bud Zn concentration during any winter following the sprays. None of the sprays influenced shoot growth or fruiting performance, or induced phytotoxicity symptoms. The results suggest that the two tested organically complexed Zn products, sprayed multiple times at low Zn rates starting slightly before bloom, can safely provide greater amounts of phytoavailable Zn than traditional dormant season sprays of high rates of inorganic Zn salts. They also validate the need for annual application of Zn and possibly Cu nutritional sprays.