Phytophthora infestans is the causal agent of potato late blight, a devastating disease of tomato and potato and a threat to global food security. Early detection and intervention is essential for effective management of the pathogen. We developed a loop-mediated isothermal amplification (LAMP) assay for P. infestans and compared this assay to conventional PCR, real-time LAMP, and droplet digital PCR for detection of P. infestans. The LAMP assay was specific for P. infestans on potato and tomato and did not amplify other potato- or tomato-infecting Phytophthora species or other fungal and bacterial pathogens that infect potato and tomato. The detection threshold for SYBR Green LAMP and real-time LAMP read with hydroxynaphthol blue and EvaGreen was 1 pg/µl. In contrast, detection by conventional PCR was 10 pg/µl. Droplet digital PCR had the lowest detection threshold (100 fg/µl). We adapted the LAMP assay using SYBR Green and a mobile reader (mReader) for use in the field. Detection limits were 584 fg/µl for SYBR Green LAMP read on the mReader, which was more sensitive than visualization with the human eye. The mobile platform records geospatial coordinates and data from positive pathogen detections can be directly uploaded to a cloud database. Data can then be integrated into disease surveillance networks. This system will be useful for real-time detection of P. infestans and will improve the timeliness of reports into surveillance systems such as USABlight or EuroBlight.