Lineage-exclusion resistance breeding: Pyramiding of blast resistance genes for management of rice blast in India

By MGR-DNA fingerprinting analyses, we identified 29 lineages of Magnaporthe grisea among rice blast pathogen population in Southern India. The population structure of M. grisea in different states of India was also characterized by a Pen-based rep-PCR method. The latter method identified I to 22 bands in the fingerprints of rice-infecting and less than 10 bands in the fingerprints of non rice-infecting strains of M. grisea which were in the size range of 0.4 to 23 kb. Lineage-exclusion assays showed that a combination of Pi-1 and Pi-2 genes for blast-resistance can exclude all the lineages identified. This virulence characteristic of M. grisea remained stable while its lineage-composition showed drastic changes over the period 1993-97 in rice-growing areas of Kerala where annual blast epidemics is common. Therefore, gene pyramids of Pi-1/Pi-1b+Pi-2 were constructed in the CO39 (blast-susceptible) genomic background and such pyramids were resistant to blast in hot-spot locations. The CO39 pyramid now selves as the R-donor for introgressing the Pi-1+Pi-2 genes for blast resistance into cultivars such as Jyothi and IR50 which are high-yielding but blast-prone to make them durably resistant.