ASHS 2025 Conference

Comparing the Efficiency of Different Foliar-Applied Zinc Formulations in Pistachio Zinc (Zn) deficiency is a major concern in California pistachio production. Foliar fertilization is a rapid and efficient way to improve Zn in pistachio trees during periods of high demand. However, the absorption and translocation of different foliar-applied Zn in pistachio trees vary significantly. Thus, it is necessary to optimize foliar Zn applications. The goals of the research were to evaluate different sources of Zn (Zn sulfate - ZnS, Zn sulfate potassium (K) 1% - ZnS K, and chelated Zn - CHZn) on leaf uptake, yield, and nut quality (fresh and dry weight, nut split in-shell, and nut non-split). The Zn dose of 807 g ha-1 was applied to 4 replications (5 trees per replicate) per treatment (including an untreated control) in a randomized complete block in a commercial ‘Golden Hills’ pistachio orchard in California. Trees were sprayed in April, a period of highest absorption efficiency (1/2- to 2/3-leaf expansion). Leaf samples were taken 2, 8, 24 hours, and 7 days after spray to measure Zn absorption efficiency. Leaf (August) and fruit samples (harvest) were taken for quality and nutrient analysis. Yield was determined for every replicate at harvest. After verifying the significance of the variance analysis (p < 0.05), the means were compared by the Duncan test (p < 0.05). Trees treated with Zn during leaf expansion exhibited significantly higher leaf Zn content than those in the control treatment. The first-year results indicated that adding K may increase the average Zn content in leaves compared to trees sprayed with Zn alone, suggesting that K at 1% may enhance Zn uptake at this stage of leaf development. Foliar application of CHZn significantly increased Zn content in leaves compared to all other treatments. Spraying Zn when leaves were between half and two-thirds expanded allowed sufficient absorption to maintain significantly higher leaf Zn contents until August, compared to control trees. As expected, no significant differences were observed in yield and nut quality in the first year of the experiment. Future research should address the cumulative effect of different Zn sources on the retention of floral buds, bud break, and flowering.