Inheritance and mechanisms of resistance to tebuconazole, a sterol C14-demethylation inhibitor, in Nectria haematococca

Genetic control and mechanisms of resistance to tebuconazole, a sterol C14-demethylation inhibitor, were investigated in the phytopathogenic fungus Nectria haematococca. Resistant mutants have been selected from the laboratory, following UV irradiation. They have been characterized through genetic crosses and mutations in at least three genes were found to be responsible for resistance. The genes Teb1, Teb2, and Teb3 were clearly identified, a fourth gene called Teb4 could be hypothesized. Mutations at loci Teb2 and Teb3 induced pleiotropic effects such as reduced sporulation and growth rate, mycelium pigmentation (Teb2), or altered ascospore viability (Teb3). The resistance levels determined by mutations in the different genes were relatively low (below 10). When associated in double mutants, the additive effect was recorded. Cross-resistance reward other sterol C14-demethylation inhibitors was observed in all the resistant strains, except in the Teb4-carrying strain; moreover, for some C14-demethylation inhibitors hypersensitivity was expressed. A constitutive energy-dependent efflux seemed implicated in the mechanism of resistance for the Teb1-carrying strain and probably also for the Teb2 and Teb3-carrying strains. However, the Teb4-carrying strain exhibited a kinetic of fungicide uptake similar to that of the wild-type strain. The sterol profile of the Teb4-carrying strain was similar to that of all the other resistant mutants and wild-type strains. Thus the resistance mechanism induced by mutation at the Teb4 locus has not been found yet, (C) 1998 Academic Press.