A native Bacillus velezensis RB6 from the passion fruit rhizosphere suppresses Phytophthora-induced root rot and promotes plant growth under nursery conditions.

Root rot caused by Phytophthora spp. is a major constraint to passion fruit production. This study evaluated indigenous rhizobacteria isolated from healthy passion fruit plants for antagonistic activity, plant growth-promoting traits, and suppression of Phytophthora root rot under nursery conditions. Ten bacterial isolates were obtained, of which RB6 showed the strongest in vitro inhibition of Phytophthora mycelial growth, together with high phosphate-solubilizing, siderophore-producing, and extracellular hydrolytic activities. The colorimetric assay for indole compounds produced an IAA-equivalent signal; because the assay was based on Salkowski reagent in tryptophan-supplemented medium, this result was interpreted as detection of Salkowski-reactive indole compounds rather than definitive confirmation of IAA biosynthesis. Multilocus analysis of gyrA and gyrB resolved RB6 as Bacillus velezensis. Across the 2,145-bp concatenated alignment, RB6 showed 100% nucleotide identity to the B. velezensis reference strains CR-502/LMG 22478ᵀ and FZB42, and the RB6-B. velezensis grouping received 100% bootstrap support. In nursery experiments, RB6 promoted seedling growth and markedly reduced disease development, root disease severity, root rot severity, and mortality compared with the pathogen control. RB6 treatment was also associated with increased peroxidase (POD), polyphenol oxidase (PPO), phenylalanine ammonia-lyase (PAL), and catalase (CAT) activities and total phenolic content following pathogen challenge. Persistence of the inoculated bacterium on passion fruit roots was independently assessed by re-isolation followed by gyrA/gyrB sequencing. Of 30 presumptive colonies recovered from RB6-treated roots, 29 (96.7%) showed the expected RB6 gyrA/gyrB sequence profile, whereas no RB6-positive colonies were detected among 30 colonies examined from uninoculated controls. Overall, these findings support B. velezensis RB6 as a multifunctional native rhizobacterium with biocontrol and plant growth-promoting potential against Phytophthora root rot under nursery conditions.