IntroductionThere is a rising prevalence of root pathogens of the genus Fusarium on coffee (Coffea arabica) farms in Saudi Arabia. The current study aimed to explore the antagonistic mechanisms involved in the interaction between four Trichoderma spp. isolated from coffee rhizospheres and two different strains of Fusarium spp., namely Fusarium oxysporum and Fusarium brachygibbosum. The main hypothesis is that the mechanism of action for the Trichoderma-Fusarium interaction involves three distinct and concurrent mechanisms, which include: (i) spatial competition due to differences in growth rates, (ii) production of antifungal metabolites, and (iii) Volatile Organic Compounds (VOC) targeting selected cellular targets within fungi.MethodsThe root system and rhizosphere soil of coffee plants from localities in Al-Baha and Jazan regions (May and August 2023) were examined for the presence of Trichoderma and Fusarium species. Six Trichoderma and Fusarium isolates were identified by molecular analysis using ITS sequencing. The antagonistic activities of the identified isolates were tested using dual culture assays, incubated at 27 °C for 7 days. The percent mycelial inhibition was calculated. VOCs released by the four Trichoderma species were extracted with ethyl acetate and analysed using GC–MS. The inhibition of fungal growth was explained by hierarchical regression, which partitioned the variance in inhibition by the growth rate of the fungi, VOC diversity, and specific compounds.ResultsThe Trichoderma strains showed 2.6–3.2x faster radial growth rate (15–19 mm/day) compared to Fusarium strains (5.9–7.9 mm/day). Trichoderma harzianum 404.1 and Trichoderma asperellum J-3 were found to produce the highest inhibition of F. brachygibbosum at 77.4 and 77.6%, respectively (p < 0.001). This was better than inhibition by the other two strains, such as Trichoderma longibrachiatum and Trichoderma koningii, which inhibited at 73.1 and 75.1%, respectively. Sixty-nine unique VOCs were identified in the four strains using GC–MS. The compounds of mechanism-related relevance included 1,8-cineole (eucalyptol), caryophyllene oxide, 6-pentyl-2H-pyran-2-one (6-PP), and 2-phenylethanol. T. harzianum 404.1 showed maximum VOC production, with 20 types, whereas T. asperellum J-3 produced only caryophyllene oxide. Therefore, our proposed mechanistic model involved all three steps: competition of Trichoderma spp. with pathogens for nutrition, emission of different VOCs to affect fungi on different fronts, and release of metabolites such as hexanedioic acid esters and phthalate derivatives, having mycolytic potential.ConclusionThese findings provide a mechanistic framework for the development of species-specific Trichoderma-based bio-fungicides and for field validation under Saudi Arabian coffee-producing conditions.