From Fermentation Optimization to Field Application: A Study on Using FoB1-Inoculated Saplings to Mitigate Apple Replant Disease

The apple industry has become a pillar industry for increasing farmers’ incomes and promoting rural revitalization. Apple replant disease (ARD), a multifactorial disease syndrome, has emerged as one of the major bottlenecks constraining the high-quality development of the apple industry. In this study, a beneficial Fusarium oxysporum strain B1 was evaluated for its potential as a biocontrol agent against ARD. Liquid fermentation conditions were optimized through single-factor experiments combined with response surface methodology to achieve labor-saving and scalable preparation of the FoB1 inoculant. Two consecutive years of pot and field experiments were then conducted in established orchards to assess the effects of FoB1-inoculated saplings on tree growth, fruit yield and quality, and the rhizosphere microenvironment under replant conditions. Optimization of the culture medium and fermentation conditions raised FoB1 biomass to 17.2 g/L, significantly higher than that before optimization, thereby laying the groundwork for scalable production. Pot and field trials consistently showed that FoB1 inoculation moderately enhanced tree vigor. FoB1 inoculation increased the first-year fruit yield and improved the fruit sugar–acid ratio at the Haiyang site. Furthermore, FoB1 treatment significantly elevated the activities of soil urease, phosphatase, and sucrase by 26.9–93.3%, 52.59–97.77%, and 45.47–79.03%, respectively, and reduced the rhizosphere population of Fusarium solani by 68.98–84.52% compared with CK, indicating that FoB1 enhances soil metabolic activity and exerts stable, sustained antagonistic effects against the main ARD pathogen. Collectively, FoB1-inoculated saplings show application potential for alleviating ARD, and this study provides a theoretical basis and technical support for developing efficient and stable green control technologies for ARD.