Two Additional Genes for Pisatin Demethylation and Their Relationship to the Pathogenicity of Nectria haematococca on Pea. Susan F. Mackintosh. Department of Plant Pathology, Cornell University, Ithaca, NY 14853-5908 U.S.A. David E. Matthews, and Hans D. VanEtten. Department of Plant Pathology, Cornell University, Ithaca, NY 14853-5908 U.S.A. . MPMI 2:354-362. Accepted 24 July 1989. Copyright 1989 The American Phytopathological Society.

The ability of the fungus Nectria haematococca to demethylate pisatin is required for its tolerance to this phytoalexin and for its virulence on Pisum sativum. Pisatin demethylase activity (Pda) is inducible by pisatin, and among Pda⁺ N. haematococca isolates there are large differences in the level of activity observed after induction. Previous analysis of two field isolates identified three genes controlling pisatin demethylation. Pda1 confers a high level of demethylase activity (Pda^SH phenotype), while Pda2 and Pda3 encode low rates of demethylation (Pda^LL). High virulence on pea was associated only with Pda1. The present study was an analysis of the control of pisatin demethylation in field isolate T-23, which expressed an intermediate level of activity (Pda^SM), and in another Pda^LL field isolate, T-161. A fourth gene, Pda4, was found to control the Pda^SM phenotype, while the Pda gene in isolate T-161 may represent another allele at the Pda3 locus. In crosses segregating for Pda4 or Pda3, only progeny with the Pda^SM phenotype were virulent; both Pda^LL and Pda¯ progeny were nonvirulent. In a cross involving Pda1 and Pda4, Pda^SH progeny were more virulent than Pda^SM progeny. Apparently the amount of phytoalexin-detoxifying enzyme produced by this fungus can directly affect its virulence on pea.