Downy mildew caused by the oomycete Peronospora sparsa affects roses and is a serious problem in nurseries and cut roses in commercial greenhouses, especially in those without heating systems. The disease, which affects the quality and the yield of roses, develops fast under suitable environmental conditions. Currently it is controlled mainly by the application of foliar fungicides and removal of symptomatic plant material due to the limited availability of resistant cultivars of high commercial value. Though downy mildew is a destructive disease and its control increases production costs, few alternatives have been explored to detect the disease at early stages. In greenhouses epidemics begin usually in localized areas of the crop. Therefore, non-invasive systems for early detection of the disease may become an alternative in a future framework of more sustainable crop management. Hence, this study evaluated the visualization of rose downy mildew in adult plants by infrared thermography for presymptomatic detection of the disease. Roses of the cultivar Elle^® susceptible to P. sparsa were inoculated, incubated and the presence and development of the disease was followed visually and thermographically. Mock inoculated plants were kept under the same conditions were used as controls. Thermal assessments were conducted using a VARIOSCAN 3201 ST camera (Jenoptic Laser, Jena, Germany) with 0.03 K thermal resolution. Initial symptoms were detected visually five days post inoculation (dpi), while presence of the disease was visualized by thermal imaging one or two days before. Results showed that leaf temperature of plants was affected after the inoculation and its dynamics changed over time depending on the phase of the disease. Infection of P. sparsa in early stages of pathogenesis was detected as an increase of leaf temperature compared to healthy tissue, which indicated a decrease in leaf transpiration. The technique allowed the recognition of infected leaflets in the leaf, affected leaves in the plant and to discriminate between healthy and diseased plants. In contrast to other host-downy mildew interactions the response of roses to leaf colonization by P. sparsa was clearly associated with an increased leaf temperature throughout the pathogen development. The use of thermography for the detection of primary foci of diseased plants at the commercial level seems to be suitable because of this unambiguous host plant response. The technology may be applied in automated monitoring systems from above the crop canopy or from the side of plant rows, even on different scales. IR thermography proved to have a high potential as a non-invasive method for the detection of the disease and has become a promising tool to be used in risk assessment programs of rose crops in commercial production systems.