Lineage-exclusion tests for blast resistance in southern India

We have identified 25 MGR-fingerprint lineages of the rice blast fungus in southern India in samplings from 1991-95; some lineages appear to be geographically limited to certain states. In artificial inoculation assays, each lineage has a definable virulence spectrum on rice cultivars that is limited by avirulence shared by all lineage members. We are testing a lineage-exclusion resistance breeding strategy, i.e., combining resistances that, in complement, exclude all lineages in the pathogen population, in contrast to the conventional strategy that combines resistances to exclude detected pathotypes. In two-semester trials in a blast nursery at Pattambi, Kerala we evaluated field performance of rice lines constructed to pyramid two dominant blast resistance genes from the near-isogenic lines C101A51 (bearing Pi-2(t)) and C101LAC (bearing Pi-lit)); previous inoculation assays indicated that no lineage in Kerala or adjoining states was compatible with Pi-2(t) and that Pi-l(t) was effective against most lineages. All pyramids remained uninfected during trials while many cultivars, including C101LAC but not C101A51, were heavily infected. Our pathogenicity survey results indicate, however, that the durability of these pyramids is better tested in other states (Tamil Nadu, Maharastra and Orissa), where we have detected isolates differentially compatible with both resistance genes. Additional pyramids of C101A51 and C103TTP (allegedly bearing a version of the Pi-1 (t) gene) were constructed. Using isolates from both India and Colombia, we demonstrated that the gene from C103TTP (renamed as Pi-1b(t)) was expressed as a recessive phenotype. We are currently working on introgressing commercial cultivars used in various regions of India with available lineage-excluding resistances.