A cost effective, simple and eco-friendly method has been developed to synthesize zinc oxide nanoparticles using Azadirachta indica A. Juss. leaf extract. Different combinations, viz., substrate concentration (0.1 and 0.2 M ZnSO4), reducing agent (2.5 and 5.0 ml leaf extract) and sunlight irradiation durations (20, 40, 60 and 80 min) were assessed for their ability of precise stabilized zinc oxide nanoparticle synthesis. Leaf extract components serve as both reducing and capping agent, while sunlight acts as a catalyst in the synthesis process. The biosynthesized zinc oxide nanoparticles were characterized by surface plasmon response, dynamic light scattering (DLS) for size and fourier transform infrared spectroscopy (FTIR) for functional group identification. The intensity and number based smallest size of biosynthesized zinc oxide nanoparticles were 84.26 and 27.85 nm, respectively with FTIR spectral analysis, indicated that the plant extract acted as the reducing and capping agents on the surface of zinc oxide nanoparticles. Antimicrobial activity of synthesized zinc oxide nanoparticles were tested against rice pathogen, Xanthomonas oryzae pv. oryzae (Xoo) and showed a significant antibacterial activity compared to control.