Huanglongbing (HLB) disease is arguably the biggest concern for citrus growers in Florida causing a drastic more than 75% decline in citrus-bearing acreage since 2005. HLB symptoms include sparse canopy, poor leaf growth and development, ultimately resulting in yield losses. Standalone applications of plant growth regulators (PGRs) and certain antibiotics have been reported to reduce the impact of HLB on tree growth and promote new flush growth. However, very little research has been done on synergistic effect of PGRs and antibiotics on tree growth and yield. The goal of HLB mitigation strategies is to promote a denser canopy, enhancing source to sink ratio as trees with dense canopies are known to produce high fruit yield. Thus, we initiated a field study to evaluate foliar applications of PGRs (auxins, cytokinins, and gibberellins) and trunk injections of Oxytetracycline (OTC) in 9-year-old orchards of Valencia on ‘Swingle’ rootstock in central Florida. This is the first research in citrus where PGRs are being applied according to tree phenology. The experimental design is a split-plot with a completely randomized block design with 8 replications. The antibiotic programs (with and without OTC injections) are assigned to the main plots, while the PGR treatments assigned to the subplots consist of an no PGR control, PGR applications with varying concentrations, and PGR applications with constant concentrations throughout the year. Parameters such as leaf area, fresh weight, dry weight, water content, chlorophyll content, starch content and carbohydrate content from leaf samples are recorded every three months starting from July 2023. Over 9 months no consistent pattern of PGR or OTC treatment was observed for vegetative traits. Nonetheless, Non-OTC trees showed higher leaf water content compared to OTC during the dry season months of October and January. Regarding yield, the study suggests that both PGR treatments, when applied with or without OTC lead to significant improvement in yield. PGR variable treatment showed 30% improvement in yield efficiency compared to both the control and PGR Constant treatments. No difference is observed between OTC and Non-OTC programs in terms of yield efficiency. These findings suggest that use of PGR treatments regardless of use of antibiotics can be beneficial for growers.
