Rice Blast in India and Strategies to Develop Durably Resistant Cultivars

Analyses of the population of Magnaporthe oryzae at Hazaribag, India, revealed high pathogen diversity influenced by host cultivars, rice ecology and geographic location even though recent evidence suggested that the population is largely clonal. Host selection of low virulence frequency and unrestricted seed movement across the country help to introduce and establish new virulence in different regions. Strategies for blast management therefore focus on accumulation of race non-specific resistance genes in commercial cultivars susceptible to the disease. We evaluated a population of rice lines containing none to six defense response (DR) genes (thaumatin, oxalate oxidase, oxalate oxidase-Like proteins, chitinase, peroxidase, HSP90) in three blast endemic locations during 2004-2006 and compared their performance with the level of resistance in monogenic lines having different Pi genes. The population was obtained by intermating advanced backcross derived lines of Vandana/Moroberekan (V4M-14-1-B with V4M-5-3-B, V4M-6-1-B and V4M-82-2-B). Disease progress curves in lines carrying five and six DR genes were comparable to the monogenic lines carrying R genes Piz and Pi9 effective at all three locations. While the monogenic lines generally exhibited an 'all or nothing effect' with high or low disease, the introgressed population had a range of disease intensities that declined progressively with the addition of each DR gene. Some defense response genes individually conferred a higher level of resistance compared to others and hence resistance was not proportional to the number of DRs present in all cases. Nevertheless, significant reduction in leaf blast intensity with increasing DRs in the introgressed lines at different locations and years suggested that accumulation of DRs conferring different mechanisms of resistance may contribute to non-specific resistance effective in multiple environments.