INFLUENCE OF MIDSUMMER BORON SPRAYS ON BORON CONTENT AND QUALITY INDEXES OF DELICIOUS APPLE

Boron (B) is required for optimal yield and quality of apple fruit (Malus domestica Borkh.) but may impair fruit quality if present in excessive amounts. A field experiment was conducted to examine the effects of a single mid-July foliar B spray (0, 11.3, 22.6 g B/tree) on the B content and postharvest quality indices of 220-gram 'Starking Delicious' apples. Fruit B concentration was positively related to B application rate and ranged from 9 to 55 mg/kg dry mass (1.3 to 7.7 mg/kg fresh mass). The relative B increases were greater in the core and inner cortex than in the outer cortex and skin, suggesting that some of the applied B entered the fruit through the tree vascular system. Increasing fruit B concentrations caused minor changes in fruit external color indices L and b and internal color index b but had no effect on firmness, soluble solids concentration, titratable acidity, starch index, external color index a, or internal color indices L and a. None of the effects were of horticultural significance. Most fruit quality indices were influenced by postharvest sampling time and reflected typical postharvest ripening patterns. The results suggest that 'Delicious' apple quality is relatively insensitive to high fruit B concentrations.