<i>Trichoderma virens</i> DM5 Against <i>Agroathelia rolfsii</i> in Tomato: A Gateway to Understanding Its Biocontrol–Biostimulant Trade-Off

<i>Agroathelia rolfsii</i> causes stem rot in multiple crops, survives as sclerotia in the absence of host plants, and remains a difficult-to-manage pathogen globally. Although <i>Trichoderma</i> species offer a sustainable biocontrol solution, their dual roles as antagonists and plant biostimulants present a complex trade-off under stress conditions that remains underexplored. This study analyzed the pathogen suppression and plant growth promotion induced by <i>Trichoderma virens</i> DM5 in the context of tomato protection against <i>A</i>. <i>rolfsii</i>. Using a combination of phenotypic assays and transcriptomic analyses, we observed that <i>T</i>. <i>virens</i> DM5 suppressed <i>A. rolfsii</i> via both mycoparasitism and antibiosis, producing antifungal compounds that inhibited pathogen growth. Simultaneously, it enhanced host vigor by increasing seed germination by 54.5% and plant survival rates by 70% under pathogen pressure. Molecular analyses revealed that plants treated with <i>T</i>. <i>virens</i> DM5 exhibited rapid early activation of defense-associated pathways, including MAPK signaling, MYB transcription factors, and amino sugar metabolism within 24 h post inoculation, followed by attenuation of prolonged defense responses. These responses reduced the need for prolonged immune activation, suggesting an energy-saving strategy, allowing the plant to allocate resources toward development and survival. These findings highlight the multifaceted role of <i>T</i>. <i>virens</i> DM5 in sustaining tomato growth and disease suppression under <i>A</i>. <i>rolfsii</i>-conducive conditions. By combining direct antagonism with strategic modulation of plant defense, <i>T</i>. <i>virens</i> facilitates an optimized biocontrol-biostimulant trade-off. Understanding this interplay is critical for enhancing the efficacy of <i>Trichoderma</i>-based applications in climate-smart, sustainable agriculture. [Formula: see text] Copyright © 2025 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.