Blast disease, caused by Magnaporthe grisea, is one of
the most devastating worldwide diseases of rice. Intro-gression of rice blast resistance genes into improved
cultivars has been considered a cost-eV ective and envi-ronmentally bene W cial means of minimizing crop losses
due to the disease. However, when the blast resistance
of a bred cultivar is based on a single resistance gene, it
can rapidly be overcome by the emergence of compati-ble races of the pathogen (Kiyosawa 1974). To prevent
disease development without such breakdown in dis-ease resistance, the use of multiline cultivars, which are
groups of near-isogenic lines (NILs) harboring various
individual resistance genes in a common background,
is considered a useful strategy for disease control in
breeding systems (Zhu et al. 2000; Koizumi 2001;
Mundt 2002; Koizumi et al. 2004)