The pathogenic capability of the tangerine pathotype of Alternaria alternata relies on the production of host-selective ACT toxin. Inoculation of A. alternata in leaves of the citrus quickly induced rapid lipid peroxidation, accumulation of hydrogen peroxide (H2O2), and cell death, indicative of host defensive response. We previously demonstrated an essential role of the A. alternata AaAP1 gene, encoding a redox-responsive YAP1-like transcription factor, to contribute to fungal pathogenicity. The AaAP1 null mutant fails to incite necrotic lesions. In this study, we show further that the fungal mutant defective at the AaAP1 locus displayed reduced activities for glutathione-S-transferase, glutathione peroxidase, glutathione reductase, and ligninolytic peroxidase, yet retained normal production of ACT toxin. In contrast to the wild-type progenitor and the genetically reverted strain, the mutant strain was unable to detoxify H2O2 effectively and was killed upon exposure to H2O2. The mutant strain induced lower levels of H2O2 accumulation in citrus leaves, compared to those induced by the wild-type or by the genetically reverted strain. Upon exposure to H2O2, A. alternata apparently changed expression of a wide array of the genes regulated by AaAP1. Thus, the impairment of the AaAP1 null mutants to incite necrotic lesions is apparently a consequence of their inability to alleviate the toxicity of ROS, and circumvention of plant defenses is important for the disease process.