Machine vision-based automatic disease symptom detection of onion downy mildew

•An image-based field monitoring system for automatically crop monitoring is developed.•Onion disease symptom was detected automatically using deep neural networks.•The model was trained using the weakly supervised learning method.•The mAP at IOU criteria 0.5 was the highest in all models from 74.1 to 87.2.•The developed system is possible to save time and cost in field crop cultivation. The effective crop management is major issue in recent agriculture because the cultivation area per farmer is increasing consistently while the aging-related reductions in the labor force. To manage crop cultivation effectively, it needs automatic monitoring in farmland. This paper presents an image-based field monitoring system for automatically crop monitoring and consists of constructing field monitoring system for periodic capturing of onion field images, training the deep neural network model for detecting the disease symptom, and evaluating performance of the developed system. The field monitoring system was composed of a PTZ camera, a motor system, wireless transceiver, and image logging module. The deep learning model was trained based on weakly supervised learning method that can classify and localize objects only with image-level annotation. It is effective to recognize crop disease symptom which has ambiguous boundary. The model was trained using captured onion images using the filed monitoring system, and 6 classes including the disease symptom were classified. The detected disease symptom was localized from background through thresholding of the class activation map. The 60% of maximum value in class activation map was determined as an Optimal threshold for disease symptom localization. Identification performance of disease symptom was evaluated using mAP metric by IoU. The results show that the mAP at IoU criteria 0.5, which should have over 50% overlap, was the highest in all models from 74.1 to 87.2. The results showed that the developed field monitoring system could automatically detect onion disease symptoms in real-time.