<p>Global food security faces increasing threats from fungal pathogens that cause significant crop yield losses. Among these, potato (<i>Solanum tuberosum</i> L.) is highly susceptible to <i>Alternaria</i> infections, leading to major economic damage and disruptions in food supply chains. Due to the limitations and risks linked to traditional fungicide use, the current study investigated the potential of <i>Trichoderma viride</i> AT85 as a sustainable biocontrol agent against <i>Alternaria solani</i>, the fungus responsible for early blight in potato (cv. Lady Rosetta). Four <i>A. solani</i> isolates were collected from various locations and analyzed for genetic diversity by RAPD-PCR utilizing primers UBC 826, 826, and 825 to identify the virulent strain. Genetic variation was observed, with isolates 1 and 2, among the five isolates, exhibiting greater infectious properties. Thirty <i>Trichoderma</i> isolates were retrieved from the soil rhizosphere, with AT85 selected for its strong inhibitory effect on <i>A. solani</i>. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry identified AT85 as <i>T. viride</i>. Gas chromatography-mass spectrometry study identified the active chemical compounds in AT85, comprising 6-Pentyl-2&#xa0;H-pyran-2-one (21%), ethanol, 2-methyl-1-propanol, and β-Caryophyllene, all recognized for their antifungal characteristics. Pathogenicity tests on potato plants indicated diverse virulence among <i>A. solani</i> isolates, with isolates 1 and 2 showing the highest pathogenicity. <i>T. viride</i> AT85 showed notable in vitro antifungal activity against <i>A. solani</i>, with larger inhibition zones at higher <i>Trichoderma</i> concentrations. Greenhouse trials evaluated the efficacy of <i>T. viride</i> AT85 as a biocontrol agent against early blight in potatoes. The foliar applications of AT85 markedly diminished the early blight disease index and decreased disease severity by 93% compared to the infected control. Foliar application significantly enhanced potato tuber weight by 45% and overall yield by 69% relative to stressed controls, particularly in the second season. Furthermore, AT85 therapy diminished stress markers, including proline and MDA in infected plants, consequently alleviating oxidative stress. Finally, the AT85 treatment enhanced the expression of defense genes <i>LOX</i> 1 and <i>PRa</i> 1, signifying the activation of induced systemic resistance and systemic acquired resistance pathways. These findings suggest that <i>T. viride</i> AT85 could be an effective, sustainable biocontrol agent for managing early blight in potatoes, providing an alternative to chemical fungicides.</p>