Synergistic effects of phosphite‐based defence induction and restoration of soil and root microbial activity on mitigating huanglongbing in sweet orange ( Citrus sinensis L.)

Huanglongbing (HLB), caused by Candidatus Liberibacter asiaticus (CLas), critically limits citrus production by disrupting root function, nutrient acquisition, phloem transport, and canopy physiology. These constraints are more pronounced in semi‐arid orchards where soil degradation intensifies disease severity and restricts recovery. This 4‐year (2021–2024) multi‐orchard field experiment evaluated an integrated management strategy combining enhanced nutrition (ENP), biochar‐driven root bioactivation (BIO), and phosphite‐mediated defence induction (DEF) to restore soil health and improve physiological resilience of HLB‐affected sweet orange trees. Treatments (Control (CON), ENP, ENP + BIO, and ENP + BIO + DEF) were applied in a randomized block design. Soil biochemical attributes, microbial biomass (PLFA), enzymatic activity, and arbuscular mycorrhizal colonization were quantified, alongside assessments of root architecture, leaf nutrient status, chlorophyll content, gas exchange, CLas titres (qPCR), disease severity, and yield. The integrated ENP + BIO + DEF treatment was best for improving soil organic carbon, cation exchange capacity, nutrient availability, soil microbial biomass and activity, and mycorrhizal colonization, while reducing bulk density. These improvements facilitated strong recovery of fine‐root traits, including greater root length, surface area, and vitality. Enhanced root function elevated leaf nutrient concentrations, photosynthetic efficiency, and chlorophyll content. CLas titres decreased by up to 96% relative to controls, with corresponding reductions in disease severity. Yield increased by 119%, accompanied by improved fruit quality indices. Economic analysis confirmed that the integrated strategy provided the highest profitability despite greater input costs. This biologically grounded framework demonstrates that simultaneous restoration of soil function, root system rehabilitation, and activation of systemic defences is essential for sustainable mitigation of HLB in degraded, nutrient‐limited citrus orchards.