The copper nanoparticles were synthesized using the leaf extract of black turmeric (Curcuma caesia Roxb.) and were characterized using UV-VIS spectrophotometer, DLS, NTA, SEM, and FTIR. The biosynthesized CuNPs showed an SPR peak in the range of 225-230 nm. The NTA results revealed the size of CuNPs to range from 100 to 200 nm. DLS measurements showed CuNPs synthesized at 300C reaction temperature produced NPs of size 116.8 nm and had a zeta potential of -21.7 mV. SEM studies revealed the morphology of copper nanoparticles as irregular rod-like structures arranged in a flower-like pattern. FT-IR analysis confirmed the presence of functional groups, viz., OH, C-H, N-H, C-N, and C=C, which triggered reduction of copper ions to CuNPs. The CuNPs showed antifungal activity against Colletotrichum capsici and showed maximum growth inhibition (63.93%) at 500 ppm concentration. The mode of action of CuNPs against the fungal pathogen was divulged by NBT staining, quantification of MDA, leakage of intracellular components, antioxidant enzymes and light microscopy. The results confirmed the generation of ROS, causing membrane lipid peroxidation, leakage of intracellular components, causing distortion and death of the fungus. The field experiment reported that CuNP successfully reduced the disease severity of leaf spot of turmeric. The treatment T6 (Rhizome treatment with CuNPs @ 100 ppm and foliar spray with CuNPs @ 100 ppm, thrice at 21 days interval) showed lowest disease severity (34.4 %) with maximum yield (20.98 t/ha). Thus, results from this study provides alternative solution for management of leaf spot of turmeric.