At the field level, biocontrol inoculants frequently exhibit inconsistent efficacy, and the cause of this is frequently unknown. Given that the biocontrol strain must compete with the local microbial community in the rhizosphere, inoculant strains with a high level of rhizosphere competence are thought to be a critical component for effective biocontrol effects. It is well recognised that a variety of factors, including plant species and soil type, affect the composition of the microbial communities in the rhizosphere. However, the existence of a pathogen can also affect the composition of the microbial population in the rhizosphere. We proposed that the rhizosphere competence of a biocontrol strain and its biocontrol impact against a soil-borne pathogen are influenced by plant species, soil type, and the pathogen. The three soil types (diluvial sand, alluvial loam, and loess loam) were kept under similar agricultural management at the same field site for 12 years as part of an experimental plot system to test the theory. We examine Pseudomonas sp. RU47's rhizosphere competence in two plant species (potato and lettuce) as well as its biocontrol activity against Rhizoctonia diseases. Plate counts were used to assess the colonisation density of an RU47 mutant that was rifampicin-resistant in the rhizosphere of both crops. Bacterial community compositions were investigated using Denaturing Gradient Gel Electrophoresis (DGGE) and 16s rRNA gene fragments amplified from entire community DNA. According to DGGE, RU47 had a more significant impact on the makeup of the bacterial population in the lettuce rhizosphere than in the potato rhizosphere. In contrast, the pathogen Rhizoctonia solani had a far greater impact on the bacterial community in the potato rhizosphere than in the lettuce rhizosphere. Only lettuce grown in alluvial soil showed a substantial influence of RU47 on the Pseudomonas-specific GacA signatures of the rhizosphere. The biocontrol properties of RU47, which are independent of soil type and plant species, as well as its negligible impact on the makeup of the local bacterial community, may be crucial considerations in the registration and use of RU47 as a biocontrol strain.
