Background <p>Leaf spot caused by <i>Alternaria alternata</i> poses a major threat to potato production, yet the distinct impacts of its key toxins remain unresolved. This pathogen releases a range of secondary metabolites that interfere with normal leaf function and accelerate disease progression. Among them, alternariol (AOH) and tenuazonic acid (TeA) stand out as potent phytotoxins, known for triggering oxidative damage and weakening host tissues during infection. In the present work, we employed a multidisciplinary approach to investigate the physiological, biochemical, histochemical, ultrastructural, and gene-expression responses of potato leaves subjected to <i>A. alternata</i> and its host-specific toxins, AOH and TeA.</p> Results <p>Histochemical assays confirmed enhanced accumulation of ROS (H₂O₂, O₂·⁻), callose deposition, and lipid peroxidation. Pathogen infection induced necrotic lesions and chlorosis, with SEM and TEM analyses revealing distinct cytological damage. <i>A. alternata</i> and TeA caused extensive tissue collapse, membrane disruption, and organelle disintegration, whereas AOH was less destructive, primarily inducing vesicle accumulation and chloroplast disorganization. This represents the first ultrastructural evidence of AOH induced cytotoxicity in plant tissue, highlighting its underestimated role in plant stress. Antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR), along with defense related enzymes including phenylalanine ammonia lyase (PAL), polyphenol oxidase (PPO), peroxidase (POX), exhibited significant induction, which was supported by Native-PAGE revealing treatment-specific isoform expression of SOD and CAT. SDS-PAGE demonstrated altered host protein profiles, with distinct induction of mid-molecular weight (40–50&#xa0;kDa) and high-molecular weight (~ 75&#xa0;kDa) proteins under toxin and pathogen stress, indicating activation of defense associated proteins. Gene expression analysis further demonstrated a clear temporal increase in <i>Cu/Zn-SOD</i>, <i>CAT1</i>, and <i>APX</i>, with the strongest activation observed at 24–48&#xa0;h, following the pattern: pathogen &gt; TeA &gt; AOH. Principal component analysis (PCA) accounted for 82.13% and 12.52% of the total variance across all measured parameters. Correlation and clustering analyses highlighted early pigment loss followed by delayed activation of oxidative stress-responsive enzymes, while exhibiting strong negative correlations between pigments and ROS, alongside positive correlations among stress-related enzymes.</p> Conclusion <p>The severity of oxidative damage followed the trend: pathogen &gt; TeA &gt; AOH. These findings reveal stress signatures and defense shifts, providing mechanistic insights into <i>Alternaria</i> pathogenesis and toxin-driven oxidative damage in potato. Advancing the understanding of host-pathogen-toxin dynamics and offering a foundation for resistance breeding in Solanaceous crops against leaf spot disease.</p> Graphical Abstract <p></p>