Abstract
Near isogenic lines of rice, Oryza sativa L., with or wrthout a gene Pi-zt for resistance to Pyricularia oryzae CAV., have been developed through backcrossings and selfings. The heterozygosity with respect to Pi-zt has been maintained during 15 generations from F1 through BC4F10 in a cross between the Malaysian cultivar Morak Sepilai and the Japanese cultivar Fujisaka 5 in which Fujisaka 5 was used as the recurrent parent through successive four backcrosses. During these generations plants were tested for resistance to a Ken 53-33 strain of the blast fungus with a wide spectrum for virulence to Japanese cultivars, and heterozygotes were selected to transmit a Pi-zt gene from Morak Sepilai to the genetic background of Fujisaka 5 (Table l). Homozygous resistant (Pi-zt/Pi-zt) plants and homozygous susceptible (+/+) plants were obtained in the BC4F11 and their progenies showed the similar agronomic characteristics under the favorable conditions free from blast disease (Table 3). They had a Pi-i gene for resistance in common that was transmitted from Fujisaka 5 (Table 4).
