Abstract Considering blast, caused by Pyricularia oryzae , as a critical threat to wheat yield and the importance of photosynthesis as the source of energy used by plants to activate their mechanisms of defence to counteract pathogen infections, this study aimed to investigate whether wheat plants compromised in their photosynthetic process by using an inhibitor of photosynthesis could become more susceptible to blast. Plants from cultivars BRS 220 (moderately resistant) and BRS Guamirim (susceptible) were non‐sprayed (‐DCMU plants) or sprayed (+DCMU plants) with a solution (10 μM) of 3‐([3,4‐dichlorophenyl]‐1,1‐dimethylurea) (DCMU) at 24 hr before inoculation with P. oryzae . Blast progressed much faster on the leaves of +DCMU plants from cultivars BRS 220 and BRS Guamirim in comparison with ‐DCMU plants of these cultivars. Increased susceptibility of plants from the two cultivars to blast caused by DCMU was associated with its negative effect on the photosynthetic apparatus, decreasing phenylalanine ammonia‐lyase activity and reducing the concentration of total soluble phenolics. Chitinase activity for the infected plants from the two cultivars was lower compared with non‐inoculated ‐DCMU plants but was greater for inoculated +DCMU plants. Distinctly, β ‐1,3‐glucanases activity was kept stable for inoculated +DCMU plants. In conclusion, manipulation of the photosynthetic process of wheat plants by using DCMU allowed to understand their response to P. oryzae infection at the physiological level and reinforced the concept that photosynthesis was important for wheat resistance to blast.