The most evident effect of foliar diseases in wheat determining reductions in biomass and yield is associated with a diminished green leaf area decreasing the cumulative absorbed radiation by the crop. However, foliar diseases as leaf rust (Puccinia triticina) reduce, not only, the radiation interception of leaves but also affect net photosynthesis rate of the remaining green area. Previous studies analyzed the effects of leaf rust on net photosynthesis rate of wheat on leaves exposed to saturating radiation levels, however,  the impact of the pathogens on photosynthesis of leaves growing under low radiation levels (i.e.: leaves of the lower part of the canopy) were not determined. Thus, this study investigated the incidence of leaf rust on photosynthesis rate of leaves exposed to different radiation levels. Two field experiments were carried out at Grignon (France, INRA-EGC station) during 2004/05 growing season and another two experiments were set in Buenos Aires (Argentina, FA-UBA) during 2005 and 2007 growing seasons. Gas exchange measurement were carried out on leaves with different disease infection levels and exposed to different light intensities. The results showed that, when considering only green leaf area, photosynthesis rate at light saturation was significantly lower on diseased than on healthy leaves. However these differences were not observed under low radiation level. Finally, although leaf rust increased dark respiration rate of green leaf area the main effect of the pathogen was the reduction of gross photosynthesis rate.