Combining Individual Protective Covers and Homobrassinolide Treatment Prolongs Tree Health and Increases Fruit Yield in Young Tango Mandarin Trees Under Endemic HLB

Huanglongbing (HLB), caused by Candidatus Liberibacter asiaticus (CLas) and vectored by Asian citrus psyllid (Diaphorina citri), remains a major constraint to sustainable citrus production. In Florida, individual protective covers (IPCs) have been adopted as an effective psyllid exclusion tool by shielding young trees from this vector of the phloem-dwelling bacterium CLas. Brassinosteroids (BRs), a class of plant steroid hormones, are being explored as a treatment to mitigate HLB and are approved for commercial use in the state. We investigated the effect of IPCs combined with homobrassinolide (HBr) applied as a foliar spray on CLas titer, canopy volume, tree growth, yield, fruit quality, and defense-related gene expression of the salicylic acid (SA) pathways in ‘Tango’ mandarin grafted on sour orange (SO) or US-942 rootstocks. After being covered with IPCs in the field for three years, trees were subjected to monthly foliar application of HBr upon IPC removal. The experiment included four treatment groups: trees with IPC and HBr spray (IPC HBr+), IPC without HBr (IPC HBr-), no-IPC with HBr (no-IPC HBr+), and no-IPC without HBr (no-IPC HBr-). IPCs effectively delayed bacterial infection for six to nine months after IPC removal, maintaining higher cycle threshold (Ct) values (lower CLas titers) than in no-IPC trees, confirming the protective effect of IPCs against early CLas colonization. The combination of IPCs and HBr spray significantly enhanced canopy volume, particularly in trees grafted on SO. This effect was sustained over one year and was consistently greater in IPC HBr+ trees than in IPC HBr- and no-IPC HBr+ or HBr- trees, suggesting a synergistic effect of the combined therapy on enhancing tree growth. The tree height and trunk diameter were primarily improved by IPC, regardless of HBr treatment. IPC-treated trees exhibited significantly greater height and trunk diameters (scion and rootstock) than no-IPC trees across one or both rootstocks, indicating that IPCs alone contribute to these horticultural growth improvements. IPC trees also showed reduced preharvest fruit drop compared to the no-IPCs trees, resulting in higher yields, with additional gains observed in IPC HBr+ trees on SO. Fruit quality attributes, including °Brix, titratable acidity, peel color, and size, did not differ significantly among treatments. Importantly, gene expression analysis revealed early and sustained upregulation of key SA pathway genes in IPC HBr+ trees, indicating that HBr effectively activated systemic acquired resistance (SAR), particularly on SO rootstock. This study highlights the complementary roles of IPCs and HBr in the management of HLB. While IPCs provided essential early protection against CLas and promoted long-term horticultural growth, HBr enhanced early canopy development, activated host defense mechanisms, and enhanced yield. The integration of both approaches offers a sustainable and effective strategy to protect young citrus trees, delay CLas infection, and improve tree health and productivity under endemic HLB.