Removal of diseased crops is a necessary agricultural operation on which a lot of labor and time will be spent every year. Augmented Reality (AR) can provide auxiliary information to help identify if a crop is infected or when to harvest, thus reducing labor, time consumption, and crop loss. In this study, we propose a crop tagging method by aerial photography with drones and AR applications. Aerial images with location information taken by a drone equipped with Real-Time Kinematic Global Navigation Satellite System (RTK-GNSS) are used to estimate the location of diseased crops. Based on the location data, the AR application runs on the mobile device will generate guidance information. The AR application will track mobile devices for real-time 3D registration. Experiments in outdoor environments verified its accuracy and feasibility.