Mmbaga, M. T., Kim M.-S., Mackasmiel, L. and Klopfenstein, N. B. 2015. Differentiation of Corynespora cassiicola and Cercospora sp. in leaf-spot diseases of Hydrangea macrophylla using a PCR-mediated method. Can. J. Plant Sci. 95: 711-717. Corynespora cassiicola and Cercospora sp. have been identified as the most prevalent and destructive leaf-spot pathogens of garden hydrangea [Hydrangea macrophylla (Thunberg) Seringe] in the southeastern USA, but they are often difficult to accurately detect and distinguish because they often occur together in a disease complex with other pathogenic leaf-spot fungi and produce very similar symptoms. This study was conducted to provide diagnostic PCR primers for detecting and distinguishing Corynespora cassiicola and Cercospora sp. among other leaf-spot pathogens of garden hydrangea. Two primer pairs showed specificity to Corynespora cassiicola and one primer pair showed specificity to Cercospora sp., and these primers did not amplify DNA from any other common fungal pathogens associated with hydrangea leaf-spot diseases. Results from this study show that DNA-based diagnostic primers provide a useful tool for pathogen detection/identification in hydrangea leaf-spot disease, which is an essential step toward understanding disease etiology and developing/applying appropriate disease-management practices in the southeastern USA.