Indigenous plant growth-promoting rhizobacteria for cocoa growth enhancement and Phytophthora pod rot management in coastal Andhra Pradesh, India.

Abstract

Plant growth-promoting rhizobacteria (PGPR) are beneficial microbes that support plant development through mechanisms such as nutrient solubilization, phytohormone production, and antagonism against phytopathogens. This study focused on isolating and characterizing PGPR strains native to the cocoa rhizosphere in the coastal region of Andhra Pradesh and assessing their efficacy against Phytophthora palmivora, the causal agent of cocoa pod rot. A total of 50 bacterial isolates were initially screened based on hydrolytic enzyme production, siderophore activity, phosphate solubilization, and antagonistic potential. Six promising isolates NEG27, NWG12, NSK3, NEG14, NEG16, and NEG3-showed strong antifungal activity and were identified through 16 S rRNA sequencing as Pseudomonas fluorescens (NEG27, NEG3), Stutzerimonas stutzeri (NWG12), and Pseudomonas otitidis (NEG14, NEG16, NSK3). In vitro antagonism assays demonstrated that NEG27 inhibited P. palmivora mycelial growth by 82.26%, followed by NWG12 (77.41%) and NEG14 (76.30%), with microscopy revealing severe hyphal disintegration and malformed zoospores. PGPR seed treatment significantly enhanced cocoa germination rates (35.71% increase over control), seedling vigour index (by 441.84%), and root-shoot biomass (p < 0.05). Field trials conducted over two cropping seasons (2022-23 and 2023-24) revealed that combined foliar application of NEG27, NWG12, and NSK3 at 1.5% significantly (p < 0.05) reduced pod rot disease severity compared to untreated controls, while also improving yield-related traits and the benefit-to-cost (B: C) ratio. These results highlight the potential of indigenous PGPR strains as sustainable biocontrol agents and viable alternatives to chemical fungicides.