The pathogenicity variation mutant PX1 that had lost pathogenicity to cultivar Tsuyuake was obtained by screening a REMI transformant library of Magnaporthe grisea P131 strain. The mutant has no difference with wild type P131 in conidia germination,appressorium formation and other biological characteristics(including sporulation capacity and colony morphology)except significantly reducing appressorium penetration on onion epidermis. Genetic analysis revealed that the mutation phenotype was cosegregats with the insertion marker, and that the mutation is due to the marker insertion which can be used for cloning the gene.