Abstract The phytopathogenic fungus Alternaria brassicicola is the causal agent of black spot disease of Brassica plants and produces a host-specific protein toxin named AB-toxin. AB-toxin is released from spores germinating only on host leaves. In this study, we found a factor for inducing AB-toxin production that is released from host leaves after spore germination has begun. GC and GC–MS analyses of the compound purified by gel filtration chromatography demonstrated that it is an oligosaccharide of 1.3 kDa. This is the first evidence that host-specific toxin production by germinating spores of a fungal pathogen is induced by recognition of host-derived factors.