RESUMEN
The resistance to rice blast disease in the Vietnamese traditional rice cultivar 'Chiembac' was studied. The blast resistance spectrum in 'Chiembac' and 15 rice differentials carrying different known resistance genes was identified using 25 Pyricularia grisea isolates derived from 15 AFLP lineages from the North, Center and South of Vietnam. None of the differential lines carrying a single resistance gene could effectively control all tested Vietnamese blast isolates. 'Chiembac' showed a different resistance pattern compared to that of the differential lines. A cross between 'Chiembac' and 'CR203', an improved rice cultivar, was made and the F2 population was used for characterization and mapping of the resistance genes in 'Chiembac'. Genetic analysis showed that the resistance against two representative isolates from two predominant lineages, VT7 and 12, in 'Chiembac' was controlled by the single dominant genes Pi-VT7 and Pi-I2. The resistance gene Pi-VT7 was closely linked to Pi-I2 and was mapped to chromosome 12 using the framework mapping population 'IR64' x 'Azucena' of 124 double haploid progenies. The resistance to the Vietnamese blast isolate VT7 in 'IR64' was also studied. The latter was controlled by one locus with major effect located on chromosome 12 and mapped closely to the AFLP marker NIN080, which was also tightly linked to the resistance gene Pi-VT7 in 'Chiembac'. Thus, the resistance locus Pi-VT7 and the resistance locus in 'IR64' probably belong to a cluster of resistance genes.