Resistance to M. grisea is a classical gene-for-gene system where a major R gene, called Pi genes in rice, is effective in preventing the infections by M. grisea strains containing the corresponding avirulence gene (Flor 1971, Silue et al. 1992). Operationally, an R gene is often identified from landrace cultivars or wild relatives of rice cultivars using differential physiological races of M. grisea (Tanksley and McCouch 1997). Currently, identification and subsequent incorporation of an R gene into adapted germplasm is a slow process because of the following classical problems in the blast system: (1)