Zinc sulfate, sugar alcohol zinc sprays improve apple quality

Researchers sprayed 'Gala' and 'Fuji' apple trees in China with zinc sulfate (ZnSO4) and sugar alcohol zinc separately during four different developmental stages: 2 weeks before budbreak, 3 weeks after bloom, the termination of spring shoot growth, and 4 weeks before harvest. They then harvested the apples at maturity and analyzed the fruit for quality and zinc concentration.

According to the report, zinc sprays during the four different developmental stages increased zinc concentration of peeled and washed fruit at harvest without phytotoxicity. The treatments 3 weeks after bloom and 4 weeks before harvest increased average fruit weight of both 'Gala' and 'Fuji' apples. Other combinations of treatments were determined to increase fruit firmness, soluble sugar, and vitamin C levels in the cultivars. The scientists said that the effects of sugar alcohol zinc applications were equal to and "more pronounced" than those of ZnSO4.

"Although the apple trees showed no zinc deficiency symptoms and the leaf zinc nutrition was at a low level, continuing zinc sprays on these trees was required to increase fruit quality," the researchers said. "We found that a single spray of sugar alcohol zinc was equal to or more effective than zinc sulfate at being absorbed by apple fruit tissue and improving fruit quality for trees grown under field conditions."

These experimental results offer new strategies for apple growers. Based on a cost analysis, the authors recommend that growers spray ZnSO4 or sugar alcohol zinc, with 0.1% zinc and 0.04% nitrogen in the spraying solution, on the abaxial/adaxial surfaces of apple leaves to runoff at 4 weeks before harvest.