Foliar biostimulants with nano-chelated Zn-B-Si reduce rind disorders and improve yield and fruit quality in ‘Washington’ Navel orange

Preharvest rind disorders, particularly fruit cracking and creasing, are major causes of yield loss and export downgrading in ‘Washington’ Navel orange. These disorders are associated with impaired peel calcium, boron, and silicon nutrition, cell-wall degradation, and rind turgor imbalance. Although biostimulants and nano-nutrients have shown individual benefits in fruit crops, their combined effectiveness against citrus rind disorders has not been systematically evaluated or compared with the industry-standard Ca + GA₃ program. This two-season field study (2024–2025) evaluated melatonin, γ-aminobutyric acid (GABA), and 24-epibrassinolide (EBR), applied individually or combined with a nano-chelated Zn-B-Si formulation, together with a standalone nano-silicon treatment and the conventional Ca + GA₃ program in a randomized complete block design. GABA + nano (Zn-B-Si) (T10) ranked first for most traits, and EBR + nano (Zn-B-Si) (T9) was statistically comparable to it for several traits (e.g., fruit number, fruit set and chlorophyll). Both combinations generally surpassed their individual applications and the conventional Ca + GA₃ standard in vegetative growth, yield and fruit quality across both seasons. T10 reduced fruit cracking by up to 65.5%, creasing by up to 66.8% and fruit drop by up to 53% compared with the untreated control. Biostimulant–nanonutrient co-application, particularly GABA combined with nano-chelated Zn-B-Si, is a promising candidate strategy for reducing rind-disorder incidence and improving yield and fruit quality in ‘Washington’ Navel orange. Costs, total active-ingredient input and packinghouse grading outcomes were not assessed and require further study.