BEYOND THE RHIZOSPHERE: BACTERIAL ENDOPHYTES AS SUSTAINABLE BIONEMATICIDES FOR PLANTATION CROPS

Abstract

Plant-parasitic nematodes (PPNs) impose annual global losses estimated between US$80 billion and US$173 billion, with root-knot nematodes (Meloidogyne spp.), the burrowing nematode (Radopholus similis), lesion nematodes (Pratylenchus spp.), and the reniform nematode (Rotylenchulus reniformis) forming a recurring damage complex across coffee, black pepper, cardamom, coconut, tea, citrus, and other long-lived plantation crops. Because these perennial systems cannot be rotated out of production, nematode populations accumulate over decades, interact synergistically with fungal pathogens such as Fusarium oxysporum, and progressively erode yield and plantation lifespan in ways that annual-crop management strategies are poorly suited to address. Chemical nematicides, long the default control measure, are increasingly constrained by environmental contamination, non target toxicity, and regulatory phase-outs of key active ingredients, creating an urgent demand for sustainable alternatives. This review synthesises current evidence on bacterial endophytes principally Bacillus, Pseudomonas, Serratia, and Streptomyces as a biologically grounded, tissue-internal alternative to rhizosphere-confined biocontrol agents. We examine the nematode complexes and disease biology specific to each major plantation crop; catalogue the taxonomic diversity of endophytic bacteria recovered from black pepper, coffee, coconut, cardamom, tea, and citrus; and detail the mechanistic basis of nematode suppression, spanning direct antagonism (lipopeptides, phenazines, hydrolytic enzymes, volatile organic compounds), induced systemic resistance, nutrient- and phytohormone-mediated indirect effects, and competitive niche exclusion. Crop-wise case studies including genome-guided Bacillus velezensis strains deployed across coffee and black pepper field trials illustrate both the promise and the persistent gap between laboratory efficacy and field-scale reproducibility. We further identify critical research gaps, including underexplored crops (cardamom, coconut), smallholder accessibility barriers, fragmented regulatory pathways for microbial biopesticides, and strain performance variability across agro-climatic zones. Finally, we outline future directions genome- and multi-omics-guided biomolecule discovery, nano-formulation for perennial-crop delivery, bacterial–fungal consortia, integration with rootstock resistance breeding, and region-specific biopesticide development that together chart a pathway toward field-durable, endophyte-based nematode management for sustainable plantation agriculture.