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• Trichoderma asperellum FJ035 has good biocontrol effect on cucumber Fusarium wilt. • Spatiotemporal competition between FJ035 and FOC was described. • Spatiotemporal competition between FJ035 and FOC affects cucumber health. Fusarium wilt is one of the major constraints of cucumber production worldwide and is difficult to control in production. This study selected a strain of Trichoderma asperellum FJ035 with a good antagonistic effect against Fusarium oxysporum f. sp. cucumerinum Owen (FOC) by plate confrontation test and secondary metabolites (volatile substances and non-volatile substances) antagonism tests. T. asperellum FJ035 inhibited spore germination of FOC and improved the germination potential of cucumber seeds. The spatiotemporal competition dynamics of T. sperellum FJ035 green fluorescence-labeled strain and FOC red fluorescence-labeled strain in the cucumber rhizosphere were monitored under a laser confocal microscope. The results showed that the vitality of FOC in the cucumber rhizosphere exceeded that of T. asperellum FJ035, but T. asperellum FJ035 exerted some inhibitory effects on FOC. When inoculation of T. asperellum FJ035 24 h before inoculation of FOC could significantly reduce the accumulation of FOC on the roots of cucumber, corresponding to the highest control efficiency against cucumber Fusarium wilt were 69.79% and 75.96% under hydroponic and potted conditions, respectively. T. asperellum FJ035 also significantly improved the root length, plant height, stem thickness, fresh weight, and area index of the first true leaves of cucumber seedlings, and effectively mitigating the adverse effects of FOC on cucumber growth. [ABSTRACT FROM AUTHOR]