A differential system for rice blast disease consists of standard blast isolates (Pyricularia oryzae Cavara..) and differential rice varieties (Oryza sativa L.) with known resistance genes. Monogenic lines for blast resistance have been newly developed at the International Rice Research Institute (IRRI) as international differential varieties and differential blast isolates from the Philippines were selected. Using a differential system based on the gene-for-gene theory, blast resistance genes were estimated in the IRRI-bred elite rice varieties. At least seven kinds of resistance genes -- Pi20(t), Pita, Pik^* (one of the Pik allele genes except Pik-s), Pik-s, Pib, Piz-t, and Pii or Pi3 -- were estimated in 42 rice varieties on the basis of reaction patterns to 14 standard blast isolates. The reaction patterns of the IRRI varieties were compared with those of the blast monogenic lines. To confirm this gene estimation, genetic analysis was done using segregating populations derived from crosses between the IRRI varieties and a susceptible Indica-type variety, CO 39. The BC₁F₂ populations (with CO 39 as recurrent parent) were segregated for reaction to the specific standard isolates. Furthermore, the estimated genes were confirmed by allelism test against the blast resistance monogenic lines.
As a result of the segregation analysis of 10 of 42 IRRI-bred varieties, seven genes -- Pi20(t), Pita, Pik^*, Pia, Pib, Pik-s -- and Piz-t, were identified. Genes Pia, Pib, Pik-s, and Piz-t were not estimated by reaction patterns to the blast isolates but identified by genetic analysis in some varieties. The effectiveness of the differential system, which is based on conventional methods and which does not require advanced facilities, is discussed as a fundamental tool to provide essential information to develop breeding programs for blast resistance.