Screening, Identification, and Characterization of Plant Growth-Promoting Rhizobacterium Strains from Alpine Grassland as Biocontrol Agents Against Fusarium oxysporum

Wilt and root rot diseases caused by Fusarium species can severely affect herbage plants, resulting in significant losses. Currently, the research and application of biological control methods targeting Fusarium-related diseases in herbage plants are limited. In this study, 11 plant growth-promoting rhizobacterium (PGPR) strains previously screened from alpine grassland were tested to assess their inhibitory effects on Fusarium oxysporum and their ability to produce siderophores, protease, cellulase, and amylase. Meanwhile, their plant growth-promoting properties, such as nitrogenase activity, phosphate dissolution, and IAA production, were determined. The results showed that the tested PGPR strains exhibited diverse functionalities. Nine PGPR strains demonstrated antagonistic effects on colony diameter against Fusarium oxysporum, with Bacillus subtilis gt11 showing the strongest inhibition, followed by Bacillus velezensis gt1 and Bacillus subtilis gtr1, with inhibition rates of 73.57%, 69.13%, and 67.94%, respectively. The test PGPR strains mainly comprised three genera: Pseudomonas, Bacillus, and Acinetobacter. Pseudomonas was the dominant genus. The PGPR strains displayed diverse inhibitory mechanisms. Bacillus velezensis gt1, Pseudomonas subtilis gt11, Bacillus subtilis gtr1, and Pseudomonas tritici gt6 exhibited high levels of siderophore, protease, cellulase, and amylase production, potentially inhibiting Fusarium oxysporum by degrading its cell wall through these enzyme activities. The majority of the tested PGPR strains have the characteristic of producing multiple enzymes. Considering the plant growth-promoting characteristics of nitrogenase activity (38.13–505.70 nmol C2H4/vial), phosphate solubilization (inorganophosphates ranging from 41.18 to 91.65 μg·mL−1; organophosphates ranging from 5.21 to 31.17 μg·mL−l), and IAA secretion (9.27–48.95 μg·mL−1), Bacillus subtilis gt11, Bacillus velezensis gt1, Acinetobacter calcoaceticus gt14, and Bacillus subtilis gtr1 show potential for development into biofertilizers or biocontrol agents.