Unlocking olive rhizobacteria: harnessing biocontrol power to combat olive root rot and promote plant growth

Olive trees are susceptible to various diseases, notably root rot caused by Pythium spp., which presents significant challenges to cultivation. Conventional chemical control methods have limitations, necessitating exploration of eco-friendly alternatives like biological control strategies. This study aims to evaluate the potential of rhizobacteria in managing Pythium schmitthenneri-induced root rot in olive trees. We screened 140 bacteria isolated from olive tree rhizospheres for antifungal activity against the pathogen in vitro. Twelve isolates exhibited promising antifungal activity, identified through 16S rDNA gene sequencing as primarily Bacillus, Pseudomonas, Stenotrophomonas, and Alcaligenes species. Particularly, Pseudomonas koreensis (A28 and A29), Pseudomonas reinekei (A16), and Bacillus halotolerans (A10) were the highest effective strains. Mechanistic investigations revealed positive protease production in all twelve isolates, with eight producing amylase and cellulase. Chitinase activity was absent, while five solubilized tricalcium phosphate. Furthermore, eight secreted hydrocyanic acid (HCN), ten synthesized indole-3-acetic acid (IAA), and nine produced siderophores. Variability existed in antimicrobial substance production, including bacillomycin (seven isolates), iturin (eleven isolates), fengycin (two isolates), and surfactin (three isolates). Plant growth-promoting rhizobacteria (PGPR) capabilities were assessed using canola (Brassica napus) seedlings, showing enhanced growth in treated seedlings compared to controls. Greenhouse experiments confirmed the biocontrol efficacy of P. koreensis A28 and Bacillus subtilis C6 against root rot disease. These findings suggest these strains could serve as promising tools for managing olive tree root rot, offering a sustainable alternative to hazardous agrochemicals.