Estimation of Functional Values of Field-Resistance Genes to Blast Disease in Some Rice Varieties

F3 Iines of the hybrids of some varieties and Japanese susceptible varieties were tested with two Japanese blast fungus strains, Ken 54-20 (race number, 003.0) and Ao 14-20 (337.3), to learn the mode of inheritance of blast resistance. Varieties used were as follows : Chinese varieties (Sifeng 43, Quan-wei-ai and Nan Jing 11), Korean varieties (Milyang 23, Raekyeong, Ir 338), IRRI's varieties (IR 24, IR 36 and IR 2061), Indian varieties (RP 9-3 and CR 44), and Hokuriku 129 (Habataki). Analyses after tests were carried out by cumulative frequency distribution curve method by Kiyosawa. In this study, plants were divided into three groups, r, m and s, based on the degree of resistance, and resistance genes were divided into two. One was the major gene which had a functional value of 1 and the other was the minor gene which had a functional value lower than 1 in a segregation of r : (m+s). The functional value of 1 means that plants which had its genotype show resistant reaction in 100 % of plants. The resistance of these varieties was explained by assuming some major genes and minor genes. Sifeng 43 has one major gene and one minor gene. Quan-wei-ai and Nan Jing 11 have two minor genes with their additive (or complementary) effect. It was suggested that there are minor genes controlling resistance in intermediate type (m) in Milyang 23, Raekyeong, Iri 338. IR 24 and IR 36. IR 2061 has two or more major genes and some minor genes. Resistance of Hokuriku 129 was explained by two major genes. At the least, Milyang 23, Raekyeong, Iri 338, IR 24, IR 36 and CR 44 were suggested to have Pi-b.