EFFECTS OF FOLIAR ZINC APPLICATIONS AT DIFFERENT TIMES IN THE GROWING SEASON ON TISSUE ZINC CONCENTRATIONS, FRUIT SET, YIELD AND GRADE OUT OF CULINARY APPLE TREES

The foliar application of zinc (ZnSO₄.7H₂O) to ‘Bramleys’ seedling/M9 apple trees was investigated to determine the effects on tissue mineral concentrations, fruit set, fruit yield and storage quality. There were five zinc application treatments: control (no zinc), post-harvest (i.e. before leaf fall in the previous year), bloom (i.e. between bud burst and 10% petal fall), fruit cell division (i.e. after petal fall) and all three of these growth stages (Experiment 1). The experiment was repeated in a second year on a similar orchard (Experiment 2). Bloom application of zinc increased the concentration of zinc in the flower receptacles compared to control trees, but the post-harvest application had no effect on these organs. In both experiments, applications of zinc during fruit cell division, increased zinc concentrations in fruitlets. In experiment 1, application of zinc during cell division significantly reduced fruit set and resulted in lower total and marketable yields than for the other treatments. In experiment 2, fruit sets for trees receiving any of the zinc application treatments were not significantly different from the control, and only application of zinc on all occasions significantly reduced harvest yield below those of the non-sprayed trees. In experiment 1, zinc application at bloom increased the severity of bitter pit that developed in apples during controlled atmosphere storage and this was associated with lower calcium concentrations. However, in experiment 2 zinc application at cell division increased the severity of bitter pit without an associated decline in calcium concentration. In both experiments zinc application at any time had no effect on skin colour, rots and firmness.