Resistance to M. grisea is a classi

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) 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