Occurrence of Alternaria black spot of strawberry was first reported on var.Morioka-16,Among varieties tested,var.Morioka-16 was found to be only one susceptible variety.The pathogen,however,showed pathogenicity on pear leaves which is susceptible to black spot of Japanese pear.The casual pathogen of black spot of strawberry,Alternaria alternata strawberry pathotype produced three host-specific toxins(HSts),which were chemically characterized,and named AF-toxin Ⅰ,Ⅱand Ⅲ,respectively.HSTs are considered as a primary determinant of pathogenicity,that is,interaction between the host plant and the pathogen.We here introduce briefly researchd on host-specific toxin focusing on AF-toxins.