Loci controlling partial resistance to rice blast do not show marked QTL x environment interaction when plant nitrogen status alters disease severity

• 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.