| Summary: | Clubroot is a disease of cruciferous crops that causes significant economic losses to vegetable production worldwide. We applied high-throughput amplicon sequencing technology to quantify the effect of <i>Trichoderma</i><i>harzianum</i> LTR-2 inoculation on the rhizosphere community of Chinese cabbage (<i>Brassica rapa</i> subsp. <i>pekinensis</i> cv. Jiaozhou) in a commercial production area. <i>T. harzianum</i> inoculation of cabbage reduced the incidence of clubroot disease by 45.4% (<i>p</i> < 0.05). The disease control efficacy (PDIDS) was 63%. This reduction in disease incidence and severity coincided with a drastic reduction in both the relative abundance of <i>Plasmodiaphora brassicae</i>, the causative pathogen of cabbage clubroot disease, and its copy number in rhizosphere soil. Pathogenic fungi <i>Alternaria</i> and <i>Fusarium</i> were also negatively associated with <i>Trichoderma</i> inoculation according to co-occurrence network analysis. Inoculation drastically reduced the relative abundance of the dominant bacterial genera <i>Delftia</i> and <i>Pseudomonas</i>, whilst increasing others including <i>Bacillus</i>. Our results demonstrate that <i>T. harzianum</i> LTR-2 is an effective biological control agent for cabbage clubroot, which acts through modulation of the soil and rhizosphere microbial community.
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