Southern stem rot (SSR), caused by Sclerotium rolfsii, is a soilborne disease of peanut (Arachis hypogaea L.). Fungicides control SSR but are most effective when applied prior to or during early stages of infection. Previous studies demonstrated early detection of foliar disease using spectral reflectance and thermal imaging, but few studies have used these technologies to detect soilborne disease. Our objective was to identify spectral and thermal signatures of peanut infected with S. rolfsii. In greenhouse experiments, S. rolfsii inoculated and mock-inoculated lateral stems of peanut were inspected daily for symptoms, and measurements were taken with a forward-looking infrared (FLIR) camera and a Jaz spectrometer to detect plant surface temperature and spectral reflectance, respectively. Symptoms (wilting of terminal leaflets) were first observed approximately 1 week after inoculation. Reflectance curves for leaflets on inoculated and mock-inoculated stems differed in both visible and near-infrared ranges. Reflectance at 550 nm and 790 nm was used to calculate a SSR spectral disease index (SSRI). Three days after first observation of symptoms, inoculated stems had lower SSRI and higher temperature compared to mock-inoculated stems. Results indicate it is possible to detect signatures of SSR in peanut during early stages of symptom expression using spectral reflectance and thermal imaging, and this may have applications for early disease detection in the field.