VIEW ARTICLE

Genetics

Inheritance of Resistance to Fenpropimorph and Terbinafine, Two Sterol Biosynthesis Inhibitors, in Nectria haematococca. Alexandrine Lasseron-De Falandre, Laboratoire de Phytopharmacie, Institut National de la Recherche Agronomique, route de St. Cyr, 78026 Versailles Cedex, France; Marie-Josée Daboussi(2), and Pierre Leroux(3). (2)Laboratoire de Cryptogamie, Université Paris-Sud, Centre d’Orsay, Bâtiment 400, 91405 Orsay Cedex, France; (3)Laboratoire de Phytopharmacie, Institut National de la Recherche Agronomique, route de St. Cyr, 78026 Versailles Cedex, France. Phytopathology 81:1432-1438. Accepted for publication 19 December 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-1432.

The genetic control of resistance to two sterol biosynthesis inhibitors was investigated in the fungal pathogen Nectria haematococca. After UV light irradiation, resistant mutants were selected on either terbinafine, a squalene epoxidase inhibitor, or on fenpropimorph, an inhibitor of the sterol ?¹⁴-reductase and/or ?⁸??⁷-isomerase. They were genetically characterized by random analysis of sexually produced ascospores. In seven induced mutants, terbinafine resistance resulted from mutations in a single gene (Ter1), leading to high resistance levels. All these strains exhibited a positive cross-resistance towards other squalene epoxidase inhibitors (naftifine, tolnaftate). The genetic analysis of five fenpropimorph-resistant mutants resulted in the identification of three independent genes. The Fen1 gene was recognized in four mutants. One highly resistant mutant carried mutations in two unlinked genes, Fen2 and Fen3, which showed an important additive effect. Depending on the gene or on the allelic mutation involved, the fenpropimorph concentrations causing a 50% reduction in the mycelial growth rate ranged from 2.2 to 25.3 ?g/ml. The corresponding values for the wild type strains were 0.1–0.2 ?g/ml. Single gene mutants exhibited a positive cross-resistance towards fenpropidin but not always towards tridemorph. In most Ter1 mutants and in some Fen1 mutants, resistance was not coupled with changes in characteristics such as growth rate, sporulation, and pathogenicity. The potential risk of resistance development is discussed.