Potential Benefits of Phenology-based Plant Growth Regulators for Managing Huanglongbing in Florida Sweet Oranges

Abstract

Huanglongbing (HLB), presumed to be caused by Candidatus Liberibacter asiaticus, impairs citrus tree health severely, resulting in reduced fruit yield, canopy decline, and long-term productivity losses. Hormonal imbalance is a key component of HLB progression, suggesting that plant growth regulators (PGRs) may provide short-to medium-term mitigation when applied strategically. A 3-year field study (2022–24) was conducted in a commercial Florida, USA, orchard using mature ‘Hamlin’ sweet orange (Citrus sinensis L. Osbeck) trees grafted onto Swingle citrumelo rootstock, arranged in a completely randomized design with four treatments and 10 single-tree replications per treatment. Phenology-based applications of cytokinin, gibberellic acid (GA3), and 2,4-dichlorophenoxyacetic acid (2,4-D) were evaluated for their effects on canopy growth, yield, yield efficiency, and fruit quality. Treatments included a control, cytokinin alone, cytokinin followed by GA3, and cytokinin followed by GA3 + 2,4-D. Cytokinin alone maintained the greatest canopy volume consistently across years, particularly under hurricane-related stress during the final season. Sequential applications of cytokinin followed by GA3 increased fruit yield and yield efficiency significantly during the first 2 years of the study. Across all 3 years, the treatment of cytokinin followed by GA3 resulted in a 20% greater cumulative yield compared with untreated control trees (465 vs. 384 kg per tree). In contrast, inclusion of 2,4-D did not improve yield, canopy performance, or fruit quality and may have been counterproductive under conditions in which preharvest fruit drop pressure was low. Fruit quality parameters, including total soluble solids and titratable acidity, were not affected negatively by any treatment. These results indicate that effective PGR use in HLB-affected citrus depends on aligning specific products with appropriate phenological stages and orchard conditions, supporting a targeted, purpose-driven approach to mitigating HLB-associated yield losses in Florida, USA, citrus production.