Identification of Magnaporthe oryzae Avirulence Genes to Three Rice Blast Resistance Genes

Rice blast, caused by Magnaporthe oryzae, is the most disastrous and widespread disease of rice. One of the most effective means of controlling rice blast is through the use of resistant rice cultivars, but newly developed resistant cultivars often lose their resistance after a few years of commercial production. The breakdown of resistance in rice cultivars to blast is probably due to genetic instability of avirulence (Avr)genes. To identify a genetic mechanism that governs the instability of Avr genes corresponding to the resistance (R)genes would be indispensable for developing rice cultivars with lasting resistance. To identify the Avr genes that correspond to the specific R genes Pik, Piz, and Piz-t, we determined the mode of inheritance of avirulence on three race-differential rice cultivars through a genetic cross of rice blast isolates and then identified the Avr genes based on the reaction of F3 families of rice to both the parental isolates and standard isolates harboring known Avr genes. We also constructed a partial linkage map containing the Avr genes by using random amplified polymorphic DNA (RAPD)analysis.