Abstract <p>Phytophthora foot rot, caused by <i>Phytophthora capsici</i>, remains a significant challenge in the cultivation of black pepper (<i>Piper nigrum</i> L.) worldwide. In this study, we analyzed the impact of <i>P. capsici</i> infection on photosynthetic efficiency and gas exchange in six elite black pepper genotypes under field conditions. By monitoring chlorophyll fluorescence and foliar gas exchange at 24, 48, and 72 h post-inoculation (hpi) of <i>P. capsici</i>, we observed significant disruptions in PSII function and key physiological processes. The fluorescence parameters, including <i>F</i>₀, <i>F</i>₀/<i>F</i>_m, <i>F</i>_v/<i>F</i>_m, Φ_PSII, <i>qP</i> and <i>qL</i>, showed sensitive responses to infection, effectively distinguishing tolerant genotypes from susceptible ones. The genotypes IISR-Shakti—the only released genotype of black pepper for cultivation, designated as ‘moderately resistant’ against Phytophthora foot rot and Kuruvantherivalli—the high yielding traditional cultivar of the crop, demonstrated greater resilience by maintaining higher photochemical efficiency and stable gas exchange, while genotypes such as ‘PMM’- the only reported ‘lemon scented’ genotype of black pepper and Edamalayar-III—a wild genotype of the crop experienced notable decreases in <i>F</i>_v/<i>F</i>_m, Φ_PSII, <i>P</i>_n, <i>C</i>_i, and E values. Correlation analysis revealed strong negative correlations between disease severity index (DSI) and these physiological traits. The results highlight the potential of chlorophyll fluorescence and gas exchange measurements as practical, non-invasive tools for early disease detection and screening. Notably, IISR-Shakti shows significant promise for breeding programs aimed at enhancing resistance to foot rot.</p>