Agriculture 5.0 leverages emerging technologies such as drones, Internet-of-Things (IoT), machine-learning (ML), and digital twins to enhance precision farming and improve crop health management. Drone-based monitoring combined with advanced ML algorithms and IoT sensors has become a crucial tool for real-time agricultural surveillance. This survey explores the integration of Unmanned Aerial vehicles (UAVs) in smart farming. We review various drone-mounted sensor technologies and payloads for plant health assessment and environmental monitoring. Furthermore, we analyze state-of-the-art ML techniques, including supervised, unsupervised, reinforcement, and federated learning, for plant classifications, disease detection, yield estimation, anomaly identification, and privacy preservation.