Estimation of genes in blast resistance in elite indica-type rice (Oryza sativa L.) varieties-bred at the International Rice Research Institute

The presence of blast resistance genes in the elite Indica-type rice (Oryza sativa L.) varieties bred at the International Rice Research Institute (IRRI) was estimated based on a differential system using Philippines isolates of the rice blast fungus Pyricularia grisea (Cooke) Sacc., according to the gene-for-gene theory. Based on the presence of the three resistance genes, Pi20, Pita and one of the Pik alleles (other than Pik-s), the 42 varieties were classified into seven groups. A group that did not harbor these three genes contained seven varieties derived from the progenies of the hybrids with IR24 as a parental variety. The largest group harboring Pita consisted of 17 varieties, including IR36 and its sister lines or progenies. The group harboring by Pi20 had seven varieties that included IR8, IR24 and their hybrid progenies. Thus, most of the IRRI varieties were classified into these three groups that included IR8, IR24, IR36 or their hybrid progenies in their pedigree. The presence of a total of seven resistance genes Pib, Pita, Piz-t, Pi20, Pik-s, one of the Pik alleles (other than Pik-s) and one of the two genes, Pii or Pi3, was estimated in these varieties. In some cases, the presence of genes like Pib, Pik-s and Piz-t could not be confirmed due to the masking effect of Pita, Pik allele, or Pi20. The number and kind of blast resistance genes in IRRI varieties were limited compared with previously reported blast resistance genes. Since the presence of Pik-s and another Pik allele was estimated in 17 varieties belonging to five groups, and that of Pib in 38 varieties belonging to four groups, it appeared that these genes were widely distributed in IRRI-bred varieties