The quinoa downy mildew is one of the major diseases affecting quinoa in Ecuador. Considering the lack of standardized methods to quantify this disease on leaves, the aim of this study was to develop a standard area diagram to quantify the severity of the quinoa downy mildew. To standardize the assessment, we developed two sets of standard area diagrams. To validate the diagram, twelve raters estimated the severity of 50 scanned leaf images at different severity levels. The proposed diagram with linear increments provided good levels of accuracy and precision, constituting thus a useful tool in epidemiological studies and for the development of strategies to control the quinoa downy mildew.