Plant disease susceptibility is often increased by nitrogen (N) application. Therefore, it is important to know if resistance loci are effective in different plant N environments. One-hundred lines of the Bala x Azucena rice (Oryza sativa) mapping population were grown in two N treatments and tested for partial resistance to blast (Magnaporthe grisea) isolate CD100. Disease severity (DS), the number and size of lesions and plant N and C concentrations were measured and the results subject to quantitative trait loci (QTL) and QTL x environment analysis. There was a 66% higher plant N concentration in the high N treatment and DS increased significantly, mostly as a result of increased numbers of lesions. Nine regions contained QTL for disease traits but only one showed evidence of statistically significant QTL x treatment interaction. This was a large effect quantitative trait locus at marker R1933 on chromosome 12 which was less effective at high N. Apparently, blast disease is increased by higher plant N, but the efficacy of partial resistance genes is not greatly affected by N application.