Different Hypotheses for Resistance Loss of Rice Varieties to Magnaporthe oryzae

It is widely believed that a rice variety loses its resistance to rice blast after three to five years in commercial production due to the emergence of new Magnaporthe oryzae pathotypes(or physiological races)(Ou,1985).However,we believe that a resistant rice variety loses its resistance to M.oryzae not due to the emergence of new physiological races.We propose that pathogenic physiological races already exist in the natural environment,serving as potential physiological races.It is only because of the lack of appropriate environmental conditions,particularly the lack of a suitable rice host,on which such a race is unable to propagate widely and exists as an inferior race.However,when there is an appropriate rice host,the potential pathogenic physiological race can quickly proliferate to become the dominant physiological race,thus leading to the loss of blast resistance in resistant rice varieties.