Exserohilum turcicum is the causal agent of northern leaf blight (NLB) disease in sorghum and maize. Early detection of this economically important pathogen is essential for effective disease management to limit yield losses. Here we present a real-time quantitative PCR (qPCR) assay specific for E. turcicum detection and biomass quantification in sorghum and maize. In planta fungal quantification was achieved through amplification of a cytochrome P450 oxidoreductase (cpr1) gene fragment and subsequent normalisation to the host glutathione S-transferase III gene (gst3). The assay could specifically detect E. turcicum in sorghum and maize, but the cpr1 gene fragment was not amplified in non-target fungal pathogens. Application of the assay with NLB diseased sorghum and maize leaf material revealed a significant increase in E. turcicum DNA in leaves with lesion symptoms when compared to leaves with early stage chlorotic fleck symptoms in both hosts. Furthermore, E. turcicum was detected at levels as low as 1 pg in infected sorghum and maize leaves. The assay enables rapid detection and quantification of E. turcicum in sorghum and maize and has useful applications in crop breeding programmes and disease management where cultivar selection and early detection of the pathogen are essential to limit disease spread.