Synergistic Nematicidal and Physiological Effects of Combined Extracts of Millettia pachyloba and Melia azedarach in Banana Challenged by Meloidogyne incognita

Abstract The root-knot nematode Meloidogyne incognita severely threatens banana production by impairing root function and overall plant vigor. In this study, a combined aqueous leaf extract of Millettia pachyloba and Melia azedarach (MPMA) at a 1 : 1 ratio (v/v) was evaluated for nematode suppression and oxidative stress alleviation under in vitro and greenhouse conditions. Using single extract comparators at equivalent total doses, the mixture consistently exceeded the highest single agent (HSA) benchmark for key endpoints, including in vitro J2 mortality, root galling, and plant nematode density. Interaction model analyses showed endpoint-dependent responses: in vitro Bliss independence and Loewe/Chou–Talalay analyses indicated mainly additive or near-additive effects, with ΔBliss values ≤ 0 and combination indices close to 1, whereas in-planta Bliss analysis showed a mild positive interaction signal at intermediate to high doses. Greenhouse experiments revealed that MPMA significantly reduced root galling, rhizosphere and root nematode populations, and improved host physiology by lowering ROS, O₂⁻, and •OH, while enhancing ascorbate peroxidase, glutathione reductase, and the reduced-to-oxidized glutathione ratio. These changes resulted in greater biomass accumulation and higher composite growth indices in infected plants. Correlation analyses demonstrated strong negative relationships between nematode burden and antioxidant defenses, and positive correlations between antioxidant status and growth performance. Overall, MPMA provided reproducible benefits beyond the best single extract under the HSA benchmark, with quantitative interaction metrics supporting an additive to near-additive interpretation in vitro and mild, endpoint-dependent synergy under greenhouse conditions. These findings underscore its potential as a practical botanical strategy for integrated nematode management in banana cultivation.