<p><i>Ficus religiosa</i> L., commonly known as peepal tree, is vulnerable to leaf spot disease caused by <i>Alternaria alternata</i> (Fr.) Keissl. This study investigates the impact of <i>A. alternata</i> infection on the photosynthetic efficiency of <i>F. religiosa</i> using chlorophyll <i>a</i> fluorescence (ChlF) analysis, which provides a typical ‘OJIP curve’ when fluorescence data are plotted on a logarithmic scale. The curve is characterized by four distinct phases: O (at 50 µs), J (2 ms), I (30 ms), and P (represents maximum fluorescence), each corresponding to different time points. ChlF measurements revealed significant distortions in the OJIP transient curve in infected leaves, indicating stress-induced alterations in photosynthetic performance. Prior appearance of visible symptoms, all non-symptomatic leaves (NSL) exhibited a marked reduction in quantum yields of electron transport (ΦE₀) and distortion in JI and IP peaks. High sensitivity of photosynthetic parameters i.e. fluorescence values at 2 ms and 30 ms, ΦE₀ and performance indexes on absorption (PIabs) and cross section (PIcs) levels against the disease reveals that these parameters can be used as physiological markers for early detection of leaf spot disease in <i>F. religiosa</i> caused by <i>A. alternata</i>. Our results also suggest that repetitive detection of photosynthetic potential through chlorophyll <i>a</i> fluorescence analysis and a JIP-test can be used as a tool for the early detection of ongoing infections whose related symptoms are still not visible on the host.</p>