Objectives Trichoderma strain TB2 and alginate oligosaccharide (AO) have the effect of improving soil microbial community. We conducted field study to explore their effect on fruit quality and reshape rhizosphere bacterial communities in ‘Dangshan’ pear (Pyrus bretschneideri), aiming to propose an alternative way to alleviate the decline of fruit quality caused by prolonged cultivation.

Methods A field experiment was conducted in a ‘Dangshan’ pear orchard in Suzhou, Anhui Province, where the pear trees were fifty-five-year-old. The treatments included root drenching 10 L/tree of TB2, AO, and their combination (A-TB2), taking water as the control (CK). During the mature stage of pears, 24 fruits were randomly sampled from each treatment group to determine individual fruit weight, longitudinal and transverse diameters, and soluble solids content. Simultaneously, DNA samples were extracted from rhizosphere soil and subjected to high-throughput sequencing (Illumina NovaSeq PE250) to analyze bacterial diversity and community composition.

Results All the TB2, AO, and A-TB2 treatments elevated single fruit weight significantly, the increment relative to CK were by 15.7%, 18.8% and 31.0%, respectively. Furthermore, the single fruit weight of A-TB2 treatment outperformed TB2 and AO treatment by an additional 10.3% and 13.2%, respectively. The A-TB2 treatment significantly increased the bacterial Shannon index. Both TB2 and AO treatments altered soil bacterial community structures, and the A-BT2 treatment generated a distinct community structure from them. Application of brown algae oligosaccharides significantly increased the relative abundance of Chloroflexi. Application of Trichoderma TB2 significantly increased the relative abundance of Verrucomicrobia. The A-TB2 treatment further significantly increased the relative abundances of Proteobacteria, Actinobacteria, and Bacteroidetes. Bacterial network analysis showed that two bacterial network modules (module#8 and module#10) were positively correlated with single fruit weight and soluble solid content. Multiple linear regression analysis identified Steroidobacter and Gp16 within the respective modules as key functional bacteria influencing fruit weight and soluble solids content. The variation in the abundance of these core bacteria across different treatments aligned with the corresponding differences in fruit quality.

Conclusions Both the individual application of Trichoderma and alginate oligosaccharide can increase single fruit weight and soluble sugar of ‘Dangshan’ Pear. The combined application of both can more effectively activate nutrients in rhizosphere, stimulate nutrient uptake, enrich core functional microorganisms such as Steroidomonas and Gp16. Therefore, the combined application of Trichoderma and alginate oligosaccharide not only improve fruit quality but also exits eco-friendly characteristics, making it a reliable solution for green upgrading of the aged ‘Dangshanli’ orchard.