The PDA1 gene of the filamentous fungus Nectria haematococca MPVI (anamorph: Fusarium solani) encodes a cytochrome P450 monooxygenase that detoxifies pisatin, the isoflavonoid phytoalexin produced by its host, garden pea (Pisum sativum). PDA1 is regulated by several signals in culture that may control its expression during pathogenesis of pea. It is induced by pisatin and repressed by glucose and amino acids. Deletion analysis was performed on the PDA1 promoter to define regulatory regions, using a β-glucuronidase (GUS) reporter gene fusion. The results identified a region between -287 and -429, relative to the start of transcription, that mediated repression by either glucose or amino acids in culture, independent from pisatin induction. Transformants bearing PDA1 promoter constructs displaying altered regulation in response to the different signals were used to infect pea epicotyls in order to correlate regulation in culture with that observed during pathogenesis of the host. Removal of the nutritional response region did not have a major effect on the induction of the promoter observed during growth in pea. However, induced expression in planta was lacking in a PDA1::GUS construct that lacked pisatin response in culture. These results suggest that the host-specific stimulus, pisatin, is a primary stimulatory signal for PDA1 regulation during pea pathogenesis.