The agricultural sector faces considerable challenges in the management of plant diseases that poses a substantial threat to global food security and economic sustainability. Conventional methods for disease detection and monitoring are typically labor-intensive and time-consuming and often lack real-time precision. Therefore, recent advancements in nanotechnology have facilitated the development of smart nanomaterials characterized by unique physicochemical properties which enables rapid and sensitive detection of plant pathogens. These materials include nanosensors, nanocomposites, and functionalized nanoparticles that can identify specific disease biomarkers, monitor plant health, and provide early warning signals to farmers and agronomists. This chapter explores the design principles, mechanisms, and applications of smart nanomaterials in plant disease diagnostics and monitoring systems. It also addresses current challenges and future prospects for integrating these nanotechnologies into precision agriculture with the aim to enhance crop protection, reduce pesticide use, and improve sustainability.