the instability of M. grisea avirulence genes not only reduces the reliability of the pathogenicity assays, but also reduces the durability of an R gene (Valent 1997, Kang et al. 2001, Dean et al. 2005); (2) a single R gene that masks the existence of other R genes in any given cultivar can trigger the resistant response; (3) it is often unknown how many R genes are present in any given cultivar, and how many avirulence genes are present in any M. grisea race; and (4) the information on R genes in any given cultivar is difficult to verify because quarantine restrictions that inhibit the exchange of different M. grisea races, particularly in rice-producing areas. DNA markers overcome the above constraints