Biocontrol Potential of Bacillus amyloliquefaciens PP19 in Alleviating Watermelon Continuous Cropping Obstacles

Continuous cropping obstacles (CCOs) lead to a decline in yield and quality under repeated cultivation in the same farmland. Notably, CCOs caused by fusarium wilt, autotoxicity, or imbalance in rhizosphere microbial communities reduce the productivity of watermelons (Citrullus lanatus). Considering the negative environmental impacts of conventional agrochemicals, it is necessary to evaluate the biocontrol efficiency of microorganisms. Therefore, this study aimed to investigate the biocontrol efficiency of Bacillus amyloliquefaciens strain PP19 against CCOs of watermelon so as to develop alternatives to agrochemicals. The inhibitory effect of PP19 on watermelon fusarium wilt was assessed through plate confrontation assays and field trials. The degradation and utilization of autotoxins by PP19 were examined via co-culture experiments. Additionally, 16S rRNA sequencing was employed to analyze the impact of PP19 on the rhizosphere soil microbial community of watermelon. Specifically, we analyzed the PP19 utilization of four phenolic autotoxins secreted by watermelon roots and assessed their effects on microbial diversity in the watermelon rhizosphere. Plant growth assays showed that PP19 improved the weight and quality of watermelon fruit. Although PP19 inhibited the growth of Fusarium oxysporum f. sp. niveum (Fon), the growth inhibitory effect was significantly enhanced by autotoxins produced by watermelon, including mixed phenolic, cinnamic, ferulic, and p-coumaric acids. Additionally, PP19 effectively degraded and utilized the autotoxins, and the autotoxins enhanced PP19’s swimming ability and biofilm formation. Moreover, PP19 treatment significantly enhanced the microbial diversity in watermelon rhizosphere, increased the number of beneficial bacterial genera, and decreased the number of pathogenic genera. Conclusively, these results suggest that B. amyloliquefaciens strain PP19 improves the resistance of watermelon to CCOs by effectively utilizing and degrading autotoxin, altering soil microbial community structure, and inhibiting Fon17 growth, resulting in improved fruit quality. Overall, PP19 possesses potential application as a biological control agent against CCOs in commercial watermelon cultivation.