Nonhost resistance is a phenomenon that enables plants to protect themselves against the majority of potential pathogens, and thus has a great potential for application in plant protection. We recently found that CfHNNI1 (for Cladosporium fulvum host and nonhost plant necrosis inducer 1) is an inducer of plant hypersensitive response (HR) and nonhost resistance. In this study, its functional mechanism was analyzed. CfHNNI1 was a single copy gene in C. fulvum genome. The functional ORF of the CfHNNI1 cDNA was ATG3-TAG780, which showed homology with genes encoding bZIP transcription factors. The functional ORF included in frame an inner one ATG273-TAG780, which was sufficient to induce HR in plants. CfHNNI1 induced plant HR in a dose-dependent manner. CfHNNI1-induced necrosis in NahG transgenic tomato plants was significantly stronger than that in their wild type controls. However, the necrosis in Nr and def1 tomato mutants was similar to that in their corresponding wild type plants. These data demonstrate that induction of HR and nonhost resistance by CfHNNI1 is negatively regulated by salicylic acid signalling pathway but independent of ethylene and jasmonic acid signalling pathways.