Synergistic soil biotic amendments and host-defense induction: a dual-action strategy for enhancing root vitality and yield in citrus Huanglongbing (HLB)

Huanglongbing (HLB), associated with Candidatus Liberibacter asiaticus (CLas), severely constrains citrus production in calcareous, stress-prone soils. This four-year, multi-site field study (2021–2024) evaluated a multi-component soil amendment integrating biochar, arbuscular mycorrhizal fungi (AMF), plant growth-promoting rhizobacteria (PGPR), chitosan, calcium-magnesium nitrates, fulvic acid, and brassinosteroids. The fully integrated treatment (T 6 ) significantly (p ≤ 0.05) improved soil organic carbon (172%), cation exchange capacity (67%), and AMF colonization (>60%) compared with the untreated control. CLas titer decreased from 6.2 to 3.9 log copies g −1 leaf tissue (absolute reduction of 2.3 log copies; ≈37% reduction), accompanied by reductions in disease severity indices (>55%). Root length density increased by 85% and fine-root biomass by 98%, while necrosis declined. Photosynthetic rate, leaf nutrient status, and relative water content improved significantly. Total yield increased by 67% (from 52.4 to 87.6 kg tree −1 ), and fruit quality (total soluble solids/titratable acidity ratio) improved from 8.5 to 13.8. Economic analysis indicated that despite higher input costs ($1,850 ha −1 ), T 6 generated the highest net return ($8,410 ha −1 ) with a benefit-cost ratio of 5.54, demonstrating economic feasibility for large-scale adoption. Although CLas was not eliminated, integrated soil biological management was associated with pathogen suppression and enhanced orchard resilience. These findings support soil-centered biological integration as a complementary strategy for managing HLB-affected citrus systems