Get It Vigorous! Supplemental Tree Growth Regulators and Mineral Nutrients Can Improve Sweet Orange HLB Tolerance

Huanglongbing (HLB) is the main disease affecting citrus crops, and in the absence of resistant commercial cultivars, alternative management strategies are required. This study aimed to evaluate the effect of early application of tree regulators associated with foliar nutrition on tree development, fruit quality, tolerance to HLB in sweet orange trees and vascular tissue organizations. The experiment was conducted in a commercial Citrus sinensis “Lima Verde” orchard, with asymptomatic (PCR−) and symptomatic (PCR+) tree, distributed in a completely randomized design with eight replications per treatment. The following six treatments were evaluated: negative control (asymptomatic trees, PCR−); positive control (symptomatic trees, PCR+); foliar application of gibberellic acid; foliar application of 2,4-D; foliar spraying of nutrients solution; and a combination of nutrients solution with gibberellic acid and 2,4-D. The evaluations included disease severity, canopy growth, analysis of mineral nutrients by fruit, dry mass, fruit quality, and histopathological analysis of the phloem and xylem. The integrated treatment (foliar nutrition + growth regulators) significantly reduced HLB symptom severity, promoted canopy growth comparable to healthy trees, and decreased fruit drop by 62.7% relative to the untreated HLB control. Fruit yield increased from 9.89 to 22.50 kg tree−1, representing a 127% increase, while harvested fruit number increased from 71.0 to 139.8 fruits tree−1. The combined treatment also restored fruit diameter to values similar to healthy trees, increased total fruit dry matter from 14.5 to 24.2 g, and maintained soluble solids (9.56 °Brix) close to the healthy control (10.33 °Brix). Fruit zinc concentration increased by 48% compared with untreated HLB-infected trees. Histological analyses demonstrated enhanced cambial activity, larger phloem and xylem areas, reduced starch accumulation, and improved vascular integrity in treated trees. These findings highlight the potential of integrated tree regulators and nutritional approaches as a valuable alternative for improving tree performance and tolerance under HLB prevalent conditions.