Nectria haematococca mating population (MP) VI isolates that contain the MAK1 gene are able to degrade maackiain, a chickpea (Cicer arietinum) phytoalexin, to a less toxic compound. To test the contribution of MAK1 to the virulence of N. haematococca MP VI on chickpea, the MAK1 gene was disrupted in a highly virulent Mak+ isolate or added to a weakly virulent Mak- isolate via transformation. The disruption of MAK1 decreased virulence to a moderate level, while addition of multiple copies of MAK1 increased virulence to either a moderate or a high level. These data demonstrate that maackiain detoxification is a determinant, but not the only determinant, of virulence in N. haematococca MP VI isolates capable of causing disease on chickpea. MAK1 is located on a 1.6-Mb conditionally dispensable chromosome. To ascertain if there are additional genes influencing virulence toward chickpea stems on the MAK1 chromosome, the loss of this chromosome was chemically induced in an isolate containing the disrupted MAK1 gene. Loss of the MAK1 chromosome did not reduce virulence toward chickpea stems further, thus indicating that no additional genes for virulence on this part of the host plant are located on the MAK1 chromosome.